Bump sphinx docs dependency

[jakob-keller]

Description of Change

Bump sphinx docs dependency

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.99%. Comparing base (7664723) to head (ea1a9d0).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1187      +/-   ##
==========================================
+ Coverage   86.94%   86.99%   +0.05%     
==========================================
  Files          64       64              
  Lines        5983     5983              
==========================================
+ Hits         5202     5205       +3     
+ Misses        781      778       -3     

Flag	Coverage Δ
unittests	86.99% <ø> (+0.05%)	⬆️

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