[CI] Disable xdist in PyPy

[abravalheri]

Currently the test suite is problematic on PyPy.
As advised by @blink1073 in #3112 (comment) disabling xdist may help.

I put this experiment together to see how disabling xdist would affect the total testing time.

Summary of changes

• Define pytest-xdist as a conditional dependency on setup.cfg, that is not installed in PyPy

Closes

Pull Request Checklist

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

[abravalheri]

It turns out the test suite takes around 20 min to run for PyPy on Windows and Mac without xdist.

With xdist, PyPy on Windows seems to take around 15 min and PyPy on Mac seems to take 10 min.

There is some difference but maybe it is worth to avoid problems?

[blink1073]

I'd say it is worth it if it is less flaky, but on my fork it had still failed without xdist, so I'm skeptical.

[abravalheri]

😩

Well, if it is still failing, then there is no much point on doing it. I will wait until the CI runs, but I will probably close the PR then.

Thank you very much @blink1073 for the help!

[abravalheri]

Yeap, even after disabling xdist the tests are still failing.

Hopefully #3120 will be enough.