Update requirements.txt

[wsavran]

pin sphinx version to 4.2.0. try to fix failing docs.

[codecov-commenter]

Codecov Report

Merging #159 (022792a) into master (3f44084) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #159   +/-   ##
=======================================
  Coverage   56.50%   56.50%           
=======================================
  Files          20       20           
  Lines        3235     3235           
  Branches      479      479           
=======================================
  Hits         1828     1828           
  Misses       1288     1288           
  Partials      119      119           

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