Test mypy integration with pytest

[dmontagu]

Change Summary

In preparation for integrating a mypy plugin, I've refactored the mypy tests to be called via mypy.api.run through pytest.

I see several benefits to this:

1. We can more easily validate the specific contents of mypy error messages
2. We can add mypy fails and successes without requiring updates to the Makefile
3. We will be able to perform coverage testing for the mypy plugin (if it gets merged).

Related issue number

@samuelcolvin Given this shouldn't have any effects on external usage of the library, I figured this pull request would be the right place for discussion, but let me know if you'd like me to open a separate issue.

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.rst file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #735 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #735   +/-   ##
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  Coverage     100%   100%           
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  Files          15     15           
  Lines        2723   2723           
  Branches      536    536           
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  Hits         2723   2723

[samuelcolvin]

LGTM, do you want to remove external-mypy now?

[samuelcolvin]

maybe I'm being dumb, but I thought we could remove external-mypy from Makefile completely?

[dmontagu]

@samuelcolvin Just pushed with external-mypy removed from makefile and CI