Update code to make test suite run, and update requirements

[tanitna]

the test suite has gotten stale and no longer worked, now it is updated to work against the currently valid wagtail/django/python combinations

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (b416601) 89.67% compared to head (ce59ce0) 89.68%.

Files	Patch %	Lines
src/wagtail_tag_manager/models/tags.py	0.00%	1 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #125   +/-   ##
=======================================
  Coverage   89.67%   89.68%           
=======================================
  Files          27       27           
  Lines        1385     1386    +1     
=======================================
+ Hits         1242     1243    +1     
  Misses        143      143           

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