Update dev requirements and sphinx conf

[rwedge]

Fixes #1755

Nbsphinx 0.8.7 exposed that we were setting source_suffix incorrectly so that got updated.

I left off updating to flake8 4.x, maybe tackle it when we start using black

[codecov]

Codecov Report

Merging #1760 (284df24) into main (0af409d) will increase coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1760      +/-   ##
==========================================
+ Coverage   98.63%   98.69%   +0.06%     
==========================================
  Files         138      138              
  Lines       15373    15373              
==========================================
+ Hits        15163    15173      +10     
+ Misses        210      200      -10     

Impacted Files	Coverage Δ
featuretools/tests/conftest.py	100.00% <0.00%> (+0.40%)	⬆️
...utational_backend/test_calculate_feature_matrix.py	99.45% <0.00%> (+0.65%)	⬆️
...computational_backends/calculate_feature_matrix.py	99.42% <0.00%> (+0.85%)	⬆️

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[thehomebrewnerd]

Remove commented line?

[thehomebrewnerd]

LGTM!