CI: Don't run xdist if pytest.mark.single_cpu

[mroeschke]

• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.

Appears some of the single builds are still timing out. Checking if not activating xdist for this build does the trick

e.g. https://github.com/pandas-dev/pandas/runs/5206822069?check_suite_focus=true

EDIT: The timeouts are due to pyarrow engine in read_csv since mamba is installing pyarrow=2.0.0 (conda seems to install pyarrow=6.0.0 when unpinned)

[jreback]

close/open a couple of times to make sure this doesn't affect

[jbrockmendel]

@mroeschke can you comment on the status of the CI and/or your gameplan?

[mroeschke]

My ideal CI Goals:

1. Ensure we're testing PY3.8-10 on Linux/MacOS/Windows with all required + optional dependencies (min versions + unpinned)
   1a) Ensure all tests that are supposed to run are run
2. Eliminate flaky behaviors (Have the red x represent an actual addressable failure)

Current CI Status:

1. Testing PY3.8-10 on Linux/MacOS/Windows with required + optional dependencies (partial dependencies on MacOS)
   1a) Some tests are still not running that could be run
2. Inching closer to eliminating flaky behaviors (it's usually always the optional dependencies s3/pyarrow)

TODO:

1. Finish testing alignment: CI: Inconsistencies in the CI tests #29685
2. Implement dependency conda-lock files to increase stability

[mroeschke]

Closing in favor of #46059