Issue #661: Fix python_requires package metadata in `setup.cfg`

[Avasam]

https://github.com/ronaldoussoren/pyobjc/issues/661#issuecomment-3475220092

Thank you !

I notice from that commit, shouldn't this line also be updated for consistency ?

https://github.com/ronaldoussoren/pyobjc/blob/ec575ded3f8993de1146ad774c532f3348aaf158/pyobjc-core/setup.cfg#L48

I'm guessing this wasn't changed because searching for 3.9 didn't reveal this line.