Add python 3.15 support

[haswee]

*Issue number of the reported bug or feature request: #289 *

Describe your changes
Add pystack support for python3.15.

Testing performed

• install python3.15
• create venv
• run (pystack-env-315) root@codespaces-0213cb:/workspaces/pystack# python3 -m pip install -e . to build and install latest version in venv
• run (pystack-env-315) root@codespaces-0213cb:/workspaces/pystack# pystack core <core file> on corefile

Testing results:

• python3.12 coredump file

(pystack-env-315) root@codespaces-0213cb:/workspaces/pystack# pystack core core.4308 
Error in import line from /workspaces/pystack-env-315/lib/python3.15/site-packages/sphinxcontrib_jsmath-1.0.1-py3.7-nspkg.pth: import sys, types, os;has_mfs = sys.version_info > (3, 5);p = os.path.join(sys._getframe(1).f_locals['sitedir'], *('sphinxcontrib',));importlib = has_mfs and __import__('importlib.util');has_mfs and __import__('importlib.machinery');m = has_mfs and sys.modules.setdefault('sphinxcontrib', importlib.util.module_from_spec(importlib.machinery.PathFinder.find_spec('sphinxcontrib', [os.path.dirname(p)])));m = m or sys.modules.setdefault('sphinxcontrib', types.ModuleType('sphinxcontrib'));mp = (m or []) and m.__dict__.setdefault('__path__',[]);(p not in mp) and mp.append(p)
  Traceback (most recent call last):
    File "<frozen site>", line 334, in _exec_imports
    File "<string>", line 1, in <module>
  KeyError: "local variable ''sitedir'' is not defined"
Error in import line from /workspaces/pystack-env-315/lib/python3.15/site-packages/sphinxcontrib_jsmath-1.0.1-py3.7-nspkg.pth: import sys, types, os;has_mfs = sys.version_info > (3, 5);p = os.path.join(sys._getframe(1).f_locals['sitedir'], *('sphinxcontrib',));importlib = has_mfs and __import__('importlib.util');has_mfs and __import__('importlib.machinery');m = has_mfs and sys.modules.setdefault('sphinxcontrib', importlib.util.module_from_spec(importlib.machinery.PathFinder.find_spec('sphinxcontrib', [os.path.dirname(p)])));m = m or sys.modules.setdefault('sphinxcontrib', types.ModuleType('sphinxcontrib'));mp = (m or []) and m.__dict__.setdefault('__path__',[]);(p not in mp) and mp.append(p)
  Traceback (most recent call last):
    File "<frozen site>", line 334, in _exec_imports
    File "<string>", line 1, in <module>
  KeyError: "local variable ''sitedir'' is not defined"
Using executable found in the core file: /venv/bin/python

Core file information:
state: t zombie: True niceness: 0
pid: 4308 ppid: 4096 sid: 4096
uid: 0 gid: 0 pgrp: 4308
executable: python arguments: python

The process died due receiving signal SIGSTOP
Traceback for thread 4308 [] (most recent call last):
    (Python) File "<frozen runpy>", line 198, in _run_module_as_main
    (Python) File "<frozen runpy>", line 88, in _run_code
    (Python) File "/usr/lib/python3.12/http/server.py", line 1329, in <module>
        test(
    (Python) File "/usr/lib/python3.12/http/server.py", line 1284, in test
        httpd.serve_forever()
    (Python) File "/usr/lib/python3.12/socketserver.py", line 235, in serve_forever
        ready = selector.select(poll_interval)
    (Python) File "/usr/lib/python3.12/selectors.py", line 415, in select
        fd_event_list = self._selector.poll(timeout)

• python3.15 coredump file

(pystack-env-315) root@codespaces-0213cb:/workspaces/pystack# pystack core core.22576 
Error in import line from /workspaces/pystack-env-315/lib/python3.15/site-packages/sphinxcontrib_jsmath-1.0.1-py3.7-nspkg.pth: import sys, types, os;has_mfs = sys.version_info > (3, 5);p = os.path.join(sys._getframe(1).f_locals['sitedir'], *('sphinxcontrib',));importlib = has_mfs and __import__('importlib.util');has_mfs and __import__('importlib.machinery');m = has_mfs and sys.modules.setdefault('sphinxcontrib', importlib.util.module_from_spec(importlib.machinery.PathFinder.find_spec('sphinxcontrib', [os.path.dirname(p)])));m = m or sys.modules.setdefault('sphinxcontrib', types.ModuleType('sphinxcontrib'));mp = (m or []) and m.__dict__.setdefault('__path__',[]);(p not in mp) and mp.append(p)
  Traceback (most recent call last):
    File "<frozen site>", line 334, in _exec_imports
    File "<string>", line 1, in <module>
  KeyError: "local variable ''sitedir'' is not defined"
Error in import line from /workspaces/pystack-env-315/lib/python3.15/site-packages/sphinxcontrib_jsmath-1.0.1-py3.7-nspkg.pth: import sys, types, os;has_mfs = sys.version_info > (3, 5);p = os.path.join(sys._getframe(1).f_locals['sitedir'], *('sphinxcontrib',));importlib = has_mfs and __import__('importlib.util');has_mfs and __import__('importlib.machinery');m = has_mfs and sys.modules.setdefault('sphinxcontrib', importlib.util.module_from_spec(importlib.machinery.PathFinder.find_spec('sphinxcontrib', [os.path.dirname(p)])));m = m or sys.modules.setdefault('sphinxcontrib', types.ModuleType('sphinxcontrib'));mp = (m or []) and m.__dict__.setdefault('__path__',[]);(p not in mp) and mp.append(p)
  Traceback (most recent call last):
    File "<frozen site>", line 334, in _exec_imports
    File "<string>", line 1, in <module>
  KeyError: "local variable ''sitedir'' is not defined"
Using executable found in the core file: /workspaces/pystack-env-315/bin/python

Core file information:
state: t zombie: True niceness: 0
pid: 22576 ppid: 10630 sid: 10630
uid: 0 gid: 0 pgrp: 22576
executable: python arguments: python

The process died due receiving signal SIGSTOP
Traceback for thread 22576 (python) [] (most recent call last):
    (Python) File "<frozen runpy>", line 201, in _run_module_as_main
    (Python) File "<frozen runpy>", line 87, in _run_code
    (Python) File "/root/.local/share/uv/python/cpython-3.15.0b1-linux-x86_64-gnu/lib/python3.15/http/server.py", line 1173, in <module>
        _main()
    (Python) File "/root/.local/share/uv/python/cpython-3.15.0b1-linux-x86_64-gnu/lib/python3.15/http/server.py", line 1150, in _main
        test(
    (Python) File "/root/.local/share/uv/python/cpython-3.15.0b1-linux-x86_64-gnu/lib/python3.15/http/server.py", line 1074, in test
        httpd.serve_forever()
    (Python) File "/root/.local/share/uv/python/cpython-3.15.0b1-linux-x86_64-gnu/lib/python3.15/socketserver.py", line 232, in serve_forever
        ready = selector.select(poll_interval)
    (Python) File "/root/.local/share/uv/python/cpython-3.15.0b1-linux-x86_64-gnu/lib/python3.15/selectors.py", line 398, in select
        fd_event_list = self._selector.poll(timeout)

Additional context
Add any other context about your contribution here.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.19%. Comparing base (4bcc46b) to head (16b5d20).

Files with missing lines	Patch %	Lines
src/pystack/_pystack/version.cpp	0.00%	2 Missing ⚠️
src/pystack/types.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #304      +/-   ##
==========================================
- Coverage   79.22%   79.19%   -0.04%     
==========================================
  Files          51       51              
  Lines        5550     5556       +6     
  Branches      581      582       +1     
==========================================
+ Hits         4397     4400       +3     
- Misses       1153     1156       +3     

Flag	Coverage Δ
cpp	79.19% <50.00%> (-0.04%)	⬇️
python	79.19% <50.00%> (-0.04%)	⬇️

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[haswee]

This breaks Wheels / test_wheels (3.13t) (pull_request) as cpython-freethreading is experimental in 3.13t.
As experimental cpython-freethreading is uninstalled, removing the flag breaks python3.13t test