CI: run mypy in full env

[mathause]

• Closescheck mypy at the end of some CI runs? #4881
• Tests added
• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst

I only added one run for py3.8 latest. To be entirely sure we could also check the typing py37-min-all-deps but it feels like overkill...

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Ok, looks good - the failure is expected - see #4878.

[keewis]

the workflow looks good to me, but it should probably be possible to skip it using [skip-ci]

Edit: since it will always fail right now, should we disable this workflow for now and you enable it in #4878?

[keewis]

looks good to me. Before merging, I think we should disable the CI for now, though, please enable it in #4878

[keewis]

Thanks, @mathause, looks good to me.

I wonder if we should move this into linting? That way, running the linting on autoupdate PRs would be easier. That should probably be a new PR, though, so I'm merging.

[mathause]

Thanks - just for reference - this is currently unconditionally inactive & should be activated in #4878