Fix test failures against Django `main`

[laymonage]

Fixes #851 (at least for now).

This is built on top of #850, and with a different approach for #847.

[codecov[bot]]

Codecov Report

Merging #852 (75bd650) into master (8dc5ab0) will decrease coverage by 2.33%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #852      +/-   ##
==========================================
- Coverage   95.81%   93.48%   -2.33%     
==========================================
  Files           3        9       +6     
  Lines         454      737     +283     
  Branches       96      161      +65     
==========================================
+ Hits          435      689     +254     
- Misses         12       31      +19     
- Partials        7       17      +10     

Impacted Files	Coverage Δ
taggit/managers.py	96.26% <100.00%> (+0.07%)	:arrow_up:

... and 6 files with indirect coverage changes

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[rtpg]

Thank you for the patches, I pulled these changes in (I went for removing Python 3.6 support outright, as its at EOL)