Drop EOL Python 3.7, Bump PyPy to 3.9

[hugovk]

Python 3.7 is EOL and no longer receiving security updates (or any updates) from the core Python team.

version	released	eol
3.11	2022-10-24	2027-10-24
3.10	2021-10-04	2026-10-04
3.9	2020-10-05	2025-10-05
3.8	2019-10-14	2024-10-14
3.7	2018-06-26	2023-06-27

https://devguide.python.org/versions/

[hugovk]

Also. the latest PyPy release added 3.10 and dropped 3.8:

• https://www.pypy.org/posts/2023/06/pypy-v7312-release.html

So let's test PyPy3.9 instead of PyPy3.8, because the latest Pillow release doesn't have precompiled wheels for PyPy3.8, and matplotlib doesn't for PyPy3.10, to prevent the CI trying to install the latest Pillow from source and failing on Windows, and being slow to compile for others.

And the PyPy jobs are slower, let's test those first to give them a head start so we don't need to wait around for them to finish last.