Drop Python 3.8 support, add 3.11 and 3.12 to CI matrix

Python 3.8 is EOL. Update requires-python to >=3.9 and CI matrix to
test 3.9, 3.10, 3.11, 3.12. Fixes mujoco build failures on 3.8.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: LemonPi

.github/workflows/python-package.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: [ "3.8", "3.9", "3.10" ]
+        python-version: [ "3.9", "3.10", "3.11", "3.12" ]
 
     steps:
       - uses: actions/checkout@v3

pyproject.toml

@@ -8,7 +8,7 @@ readme = "README.md" # Optional
 # 'Programming Language' classifiers above, 'pip install' will check this
 # and refuse to install the project if the version does not match. See
 # https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires
-requires-python = ">=3.6"
+requires-python = ">=3.9"
 
 # This is either text indicating the license for the distribution, or a file
 # that contains the license