Drop support for soon-EOL Python 3.7

[hugovk]

Python 3.7 reaches end-of-life on 2023-06-27:

• https://devguide.python.org/versions/
• https://peps.python.org/pep-0537/

Which comes before our next quarterly release on 2023-07-01.

Let's remove support and free up CI resources.

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Open issue:

• Amazon Linux 2 uses Python 3.7. We either need to drop it from the test matrix, or upgrade its Python

[hugovk]

• Amazon Linux 2 uses Python 3.7. We either need to drop it from the test matrix, or upgrade its Python

Now uses 3.9: python-pillow/docker-images#174. Documentation updated in this PR.