README.md: Upgrade badge for Supported Python Versions

[cclauss]

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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.

Description

Stop pinning the badge to an old version.
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[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.98%. Comparing base (45cc90b) to head (d014753).
Report is 83 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3737   +/-   ##
=======================================
  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]

Funny there's now two PRs after this went unnoticed for so long!

• docs: update supported python versions #3716

[willmcgugan]

thanks

[willmcgugan]

Thanks. Went with Tom's PR.