Explore using pytest-xdist to speed up CI jobs.

[sbs2001]

See https://pypi.org/project/pytest-xdist/ .

The github runner has 2 cores so we could possibly cut down the job time by half.

The CI time is not a big issue atm, but it will become one after we start running tests like #365 .

The pytest version we use is also outdated, so we need to

1. Use latest pytest
2. Use the xdist plugin

[sbs2001]

Fixed via 1c20128 and 5300ce9 .

There has been slight improvement in speed. Not sure whether the imporvement is masked by importers taking longer duration to run (eg redhat importer now has backoff).