Drop Django 2 support

[ryanhiebert]

Clean up the testing and some deprecations in the process.

[codecov]

Codecov Report

Merging #25 into main will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #25   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           72        71    -1     
  Branches        24        24           
=========================================
- Hits            72        71    -1     

Impacted Files	Coverage Δ
...ngo_safemigrate/management/commands/safemigrate.py	100.00% <ø> (ø)
src/django_safemigrate/apps.py	100.00% <100.00%> (ø)

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