Fix documentation of minimum supported Python version

[jakob-keller]

Description of Change

Fix documentation of minimum supported Python version, following merge of #1351.

Assumptions

None

Checklist for All Submissions

• I have added change info to CHANGES.rst
• If this is resolving an issue (needed so future developers can determine if change is still necessary and under what conditions) (can be provided via link to issue with these details):
  • Detailed description of issue
  • Alternative methods considered (if any)
  • How issue is being resolved
  • How issue can be reproduced
• If this is providing a new feature (can be provided via link to issue with these details):
  • Detailed description of new feature
  • Why needed
  • Alternatives methods considered (if any)

Checklist when updating botocore and/or aiohttp versions

• I have read and followed CONTRIBUTING.rst
• I have updated test_patches.py where/if appropriate (also check if no changes necessary)
• I have ensured that the awscli/boto3 versions match the updated botocore version
• I have added URL to diff: boto/botocore@[CURRENT_BOTO_VERSION_TAG]...[NEW_BOTO_VERSION_TAG]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.03%. Comparing base (cafe180) to head (59dc5dc).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1356   +/-   ##
=======================================
  Coverage   91.03%   91.03%           
=======================================
  Files          64       64           
  Lines        6745     6745           
=======================================
  Hits         6140     6140           
  Misses        605      605           

Flag	Coverage Δ
unittests	91.03% <ø> (ø)

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