Skip mypy install for pypy and mypy tests if it's not installed

[cdce8p]

Mypy recently added the librt dependency. Unfortunately it can't be installed on PyPy, thus causing the CI builds to fail.
It's already resolved upstream but not yet released. Once it is, this PR can be reverted. python/mypy#20384

[Pierre-Sassoulas]

I think we could add a skip if mypy is not installed, let me know if you want me to do it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.39%. Comparing base (4448906) to head (9c87714).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2905   +/-   ##
=======================================
  Coverage   93.39%   93.39%           
=======================================
  Files          92       92           
  Lines       11214    11214           
=======================================
  Hits        10473    10473           
  Misses        741      741           

Flag	Coverage Δ
linux	93.25% <ø> (ø)
pypy	93.39% <ø> (ø)
windows	93.37% <ø> (ø)

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see 3 files with indirect coverage changes

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