[BEAM-14172] Update tox.ini for pydocs

[aaltay]

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[TheNeuralBit]

I think I tried updating sphinx at some point because I thought it would be easy and it turned out it was non-trivial for some reason (not super helpful I know..)

[aaltay]

I think I tried updating sphinx at some point because I thought it would be easy and it turned out it was non-trivial for some reason (not super helpful I know..)

I looked at the version gap and did not attempt to upgrade. I think we need to re-purpose the bug to update sphinx.

[codecov]

Codecov Report

Merging #17176 (2d35d86) into master (993061a) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #17176   +/-   ##
=======================================
  Coverage   73.96%   73.96%           
=======================================
  Files         672      672           
  Lines       88259    88262    +3     
=======================================
+ Hits        65277    65283    +6     
+ Misses      21869    21866    -3     
  Partials     1113     1113           

Flag	Coverage Δ
python	83.64% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdks/python/apache_beam/typehints/decorators.py	92.73% <0.00%> (+0.06%)	⬆️
...runners/interactive/display/pcoll_visualization.py	86.36% <0.00%> (+0.50%)	⬆️
...pache_beam/runners/interactive/interactive_beam.py	81.39% <0.00%> (+0.77%)	⬆️

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[TheNeuralBit]

@youngoli you may need this on the release branch