Add workflow to regularly test PyPI install

[mdickinson]

This PR adds a workflow to regularly test an installation of Traits and its test dependencies from PyPI.

Plan:

• Get the workflow working, with trigger being "pull-request"
• Change the trigger to a cron job, running twice monthly.

[mdickinson]

Note: once wheels are published, I'll expand this workflow to also test PyPI installs from wheels.

[rahulporuri]

LGTM.

Note: once wheels are published, I'll expand this workflow to also test PyPI installs from wheels.

Does this need an issue or is that overkill?

[rahulporuri]

I'm not sure what the reference is here. Nothing actionable here, just wondering.

[mdickinson]

By "reference" you mean documentation reference?

It's following the same "crontab" format as standard Unix cron. Main docs are here. More here.

[rahulporuri]

Sorry. I mean't why at 03:27 UTC on the 8th and 22nd?

[mdickinson]

Ah, right. No particular reference. I just wanted a time that was outside normal working hours for both UK and US, and didn't match times that might be used in other cron jobs. The 8th (gah; sorry; the 22nd) was chosen so that this workflow first runs on the day after the 6.2.0 release, while I might still remember to check it.

[mdickinson]

Does this need an issue or is that overkill?

Done in #1407. I've assigned it to this milestone - the ideal is that it should run on Friday morning for the wheels that (if all goes well) were created and uploaded to PyPI after Thursday's release.