Include tox.ini in the sdist; fix stale MANIFEST and tox references

The source distribution was missing tox.ini, making downstream testing
harder (#43). Add it to MANIFEST.in.

While here, fix two stale references that worked against the same goal:
- MANIFEST.in pointed at a root-level test_httpbin.py that no longer exists
  (tests moved to tests/); replace it with `recursive-include tests *.py`
  so the suite is explicitly shipped instead of relying on auto-discovery.
- tox.ini's envlist still listed py37 and pypy3 (both dropped) and omitted
  3.13/3.14; align it with the supported CPython versions so `tox` from the
  sdist doesn't fail on unsupported interpreters.

Verified by building the sdist: tox.ini and tests/ are present, and
`tox -l` against the shipped config lists py38..py314.

Closes #43. Supersedes #63.

Co-Authored-By: Mirochill <200482516+Mirochill@users.noreply.github.com>

Author: kevin1024

MANIFEST.in

@@ -1,3 +1,4 @@
-include README.md LICENSE.ISC LICENSE.MIT AUTHORS test_httpbin.py
+include README.md LICENSE.ISC LICENSE.MIT AUTHORS tox.ini
+recursive-include tests *.py
 recursive-include httpbin/templates *
 recursive-include httpbin/static *

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py37,py38,py39,py310,py311,py312,pypy3
+envlist = py38,py39,py310,py311,py312,py313,py314
 
 [testenv]
 extras = test