Bump gunicorn to 20.0.2

[villebro]

CATEGORY

Choose one

• Bug Fix
• Enhancement (new features, refinement)
• Refactor
• Add tests
• Build / Development Environment
• Documentation

SUMMARY

This PR bumps gunicorn to the latest version, which should make Superset completely Python 3.7 compatible. gunicorn==20.0.0 had a regression which broke rendering of static pages when using gevent, but the 20.0.2 version has been tested to work well with gevent and from behind an SSL terminated LB (this didn't work with gunicorn==19.9.x).

ADDITIONAL INFORMATION

• Has associated issue: closes gunicorn==19.8.1 does not work on Python 3.7 #8349
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

REVIEWERS

@mistercrunch @john-bodley

[codecov-io]

Codecov Report

Merging #8643 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #8643   +/-   ##
=======================================
  Coverage   65.78%   65.78%           
=======================================
  Files         482      482           
  Lines       23814    23814           
  Branches     2589     2589           
=======================================
  Hits        15667    15667           
  Misses       7978     7978           
  Partials      169      169

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