workaround docs failure by requiring old Django

[n2ygk]

Fixes #803

Description of the Change

Works around tox -e docs failure by forcing old version of Django (<3.0).

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

Merging #804 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #804   +/-   ##
=======================================
  Coverage   97.45%   97.45%           
=======================================
  Files          56       56           
  Lines        3068     3068           
=======================================
  Hits         2990     2990           
  Misses         78       78           

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[sliverc]

Closing in favor of #808