Pin Gunicorn version to avoid a bug in 20.0.0

Codecov Report

Merging #542 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #542   +/-   ##
=======================================
  Coverage   84.44%   84.44%           
=======================================
  Files         209      209           
  Lines       12428    12428           
=======================================
  Hits        10495    10495           
  Misses       1933     1933

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merged

mentioned in commit 6b97a5bc