django-filter>=2.1

[n2ygk]

Follows #561

Description of the Change

Updates tox/travis to use django-filter>=2.1

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

[n2ygk]

@sliverc I'm not sure it's strictly necessary to do this but figured perhaps the test requirements should be consistent with what's in requirements-development.txt.

[codecov]

Codecov Report

Merging #656 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #656   +/-   ##
=======================================
  Coverage   96.15%   96.15%           
=======================================
  Files          57       57           
  Lines        2889     2889           
=======================================
  Hits         2778     2778           
  Misses        111      111

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[sliverc]

It doesn't really make a difference in terms of what deps are installed but doesn't really hurt either. Hence merging.