Add pytest to mypy dependencies

[dstansby]

If/when we start type checking tests, this will be needed to give mypy type information from pytest. See also #1844 (comment).

TODO:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/tutorial.rst
• Changes documented in docs/release.rst
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[d-v-b]

If/when we start type checking test

Yes and immediately :)

[joshmoore]

👍 from my side. I went looking whether dependencies need to be re-hardcoded but pre-commit/pre-commit#730 (comment) is pretty definitive:

pre-commit does not install from the repository under test. period. doing so makes caching intractable

[jhamman]

Amazing! Thank you @dstansby!