Add Python 3.12 support and drop Python 3.9

Author: vkarampudi

environment.yml

@@ -1,8 +1,8 @@
-name: s2t-env-py311
+name: s2t-env-py312
 channels:
   - conda-forge
 dependencies:
-  - python=3.11
+  - python=3.12
   - pip
   - pip:
     - --index-url https://pypi.org/simple

setup.py

@@ -105,9 +105,9 @@ def select_constraint(default, nightly=None, git_master=None):
         'Operating System :: MacOS :: MacOS X',
         'Operating System :: POSIX :: Linux',
         'Programming Language :: Python',
-        'Programming Language :: Python :: 3.9',
         'Programming Language :: Python :: 3.10',
         'Programming Language :: Python :: 3.11',
+        'Programming Language :: Python :: 3.12',
         'Programming Language :: Python :: 3 :: Only',
         'Topic :: Scientific/Engineering',
         'Topic :: Scientific/Engineering :: Artificial Intelligence',