Drop Python 3.8 support, require Python 3.9+

juliacall 0.9.28 requires Python 3.9+. Python 3.8 reached EOL in
October 2024, so dropping support aligns with upstream requirements.

Changes:
- Remove Python 3.8 from package classifiers
- Add python_requires='>=3.9' to setup.py
- Update CI matrix to test Python 3.9 and 3.13 instead of 3.8 and 3.13

This fixes the CI error:
"ERROR: No matching distribution found for juliacall>=0.9.28"
on Python 3.8 runners.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas

.github/workflows/CI.yml

@@ -19,7 +19,7 @@ jobs:
           # Disabling Windows tests as it's known to not work:
           # https://github.com/SciML/diffeqpy/pull/86#issuecomment-1011675735
           # - windows-latest
-        python-version: ['3.8', '3.13']
+        python-version: ['3.9', '3.13']
       fail-fast: false
     name: Test ${{ matrix.os }} ${{ matrix.architecture }}
       Python ${{ matrix.python-version }}

setup.py

@@ -12,7 +12,6 @@ def readme():
       classifiers=[
         'Development Status :: 4 - Beta',
         'License :: OSI Approved :: MIT License',
-        'Programming Language :: Python :: 3.8',
         'Programming Language :: Python :: 3.9',
         'Programming Language :: Python :: 3.10',
         'Programming Language :: Python :: 3.11',
@@ -28,6 +27,7 @@ def readme():
       author_email='[email protected]',
       license='MIT',
       packages=['diffeqpy','diffeqpy.tests'],
+      python_requires='>=3.9',
       install_requires=['juliacall>=0.9.28', 'jill'],
       include_package_data=True,
       zip_safe=False)