docs: update supported python versions

[TomJGooding]

Update the badge in the README for currently supported Python versions.

Also update the required Python version in the docs introduction.

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.98%. Comparing base (45cc90b) to head (c45f73a).
⚠️ Report is 216 commits behind head on master.
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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3716   +/-   ##
=======================================
  Coverage   97.98%   97.98%           
=======================================
  Files          74       74           
  Lines        8142     8142           
=======================================
  Hits         7978     7978           
  Misses        164      164           

Flag	Coverage Δ
unittests	97.98% <ø> (ø)

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[TomJGooding]

This will probably also need updating in the README translations, but it looks like some of these have become out of sync and don't currently include this badge?

[willmcgugan]

Thanks, Tom.

[TomJGooding]

No problem! What do you think about updating all the README translations also? #3716 (comment)

---

Sorry I realise you're taking a break, but on the subject of the docs, it would also be nice to get both the latest/stable versions working again. #3697 (comment)

[TomJGooding]

I've gone ahead and updated the badges in the README translations here: #3738