[CI] Attempt to capture yet another common PyPy error

[abravalheri]

In previous PRs, I have tried to improve the situation with tests being flaky on PyPy (specially via timeouts).

Still, there seems to be problems with ProcessPoolExecutor:

• https://github.com/pypa/setuptools/runs/5241255918?check_suite_focus=true#step:5:321
• https://github.com/pypa/setuptools/runs/5250475520?check_suite_focus=true#step:5:38

This might be related to the fact that pytest-xdist can struggle to run tests that use threads/processes themselves and that can be worse in PyPy. One alternative could be disabling xdist for PyPy (but I don't know how much longer the tests would take)

Summary of changes

• Intercept futures.process.BrokenProcessPool/MemoryError on PyPy and xfail

Closes

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d/.
  (See documentation for details)

[abravalheri]

The errors in PyPy are always fun: ValueError: Egg metadata expected at build/bdist.macosx-10.7-x86_64/wheel/foo-0.0.0-py3.7.egg-info but not found

[abravalheri]

At least this approach seems to have worked in this very own PR: https://github.com/pypa/setuptools/runs/5251092910?check_suite_focus=true#step:5:55