Drop support for Python 3.9

[redeboer]

Python 3.9 is reaching its EOL in October 2025:
https://devguide.python.org/versions

Implementation PRs

• BREAK: drop support for Python 3.9 #535
• BREAK: drop support for Python 3.9 ampform#482
• BREAK: drop support for Python 3.9 ampform-dpd#181
• BREAK: drop support for Python 3.9 polarimetry#429
• BREAK: drop support for Python 3.9 qrules#323
• MAINT: drop support for Python 3.9 report#34
• BREAK: drop support for Python 3.9 sphinx-api-relink#37
• BREAK: drop support for Python 3.9 sphinx-hep-pdgref#48
• BREAK: drop support for Python 3.9 sphinx-pybtex-etal-style#16
• BREAK: drop support for Python 3.9 tensorwaves#552