CI: run typechecks only once, not for each Django/Python version

[akx]

While working on #271/#272/#273, I noticed mypy/pyright typechecks were being run for all matrix combinations, which doesn't really make sense IMO.

This splits the typecheck parts out of the test CI job.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 100.00%. Comparing base (7cc1121) to head (58ed5a2). :warning: Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #274   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines          130       130           
  Branches        14        14           
=========================================
  Hits           130       130           

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[mattseymour]

So the current master branch of this project runs mypy against each version of python. This change only runs mypy against a fixed version - giving less coverage.

What advantage does this new way give the project. In the past we have had a type issue which affected an older version of the project but not newer versions. In that instance having a thorough typing check was useful.

[akx]

In the past we have had a type issue which affected an older version of the project but not newer versions. In that instance having a thorough typing check was useful.

Given type checks aren't being run at runtime, that shouldn't be a thing.

A typing syntax issue like using new types (dict[list], say) without deferred annotations enabled will show up in regular tests, which are still being run on every supported version.