Update Python requirements

Move to Python 3.9.

Python 3.8 is EOL as of 2024-10-07

Author: fxcoudert

pyproject.toml

@@ -6,9 +6,9 @@ authors = [
 ]
 description = "CoRE MOF database as a Python package"
 readme = "README.md"
-requires-python = ">=3.8"
+requires-python = ">=3.9"
 classifiers = [
-  "Development Status :: 4 - Beta",
+  "Development Status :: 5 - Production/Stable",
   "Intended Audience :: Science/Research",
   "License :: OSI Approved :: MIT License",
   "Operating System :: OS Independent",