Restore Python 3.9 as one of the default Pythons for Plone 6.0.

[mauritsvanrees]

See plone/plone.folder#54 (comment) and further comments.

[davisagli]

Should we drop support for Python 3.9 at this point? It reached end of life in October 2025.

[mauritsvanrees]

I don't want to drop it in Plone 6.0. Latest Plone 6.0.15 supports Python 3.9, so all packages should continue to support 3.9.

Here in plone.meta we more or less dropped it already from the test matrix previously.
But I now notice that this causes problems in the check-python-versions check:

check-python-versions....................................................Failed
- hook id: check-python-versions
- exit code: 1

setup.py says:          3.9, 3.10, 3.11, 3.12, 3.13
- python_requires says: 3.9, 3.10, 3.11, 3.12, 3.13, 3.14
tox.ini says:           3.10, 3.11, 3.12, 3.13

This is because in setup.py we want to have 3.9 in classifiers and python_requires, and in tox.ini the test matrix starts at 3.10.
(BTW, the extra 3.14 that is reported in python_requires is not a real problem.)