3.10: Add Trove classifier and to tox.ini, update setup.py version check and Python support table

[hugovk]

For #5569.

Changes proposed in this pull request:

• Add Trove classifier
• Add to tox.ini
• Update setup.py version check
• Update Python support table

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Also:

• Fix a typo
• Use ==== instead of ---- in tables to separate headers and body, so Sphinx renders the headers with <th> instead of <td>, also means we don't need manually add bold to headers:

->

• Add an extra "blank row" to tables in some places, to add a break and make tables narrower and fit better onscreen:

...

->

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• I'm not so sure about this one, what do you think? Split the Python support table into two so they fit on screen with no horizontal scrolling:

->

Compare:

• https://pillow.readthedocs.io/en/stable/installation.html
• https://pillow--5570.org.readthedocs.build/en/5570/installation.html