Add tests requirement to publish workflow in CI

[dhomeier]

Description

To let the publish job only run after all envs in tests have completed successfully; to allow this to actually happen added allowed_failures to the (mostly PySide2) envs that are failing or segfaulting more or less erratically.
Also adding a codecov.yml to keep it from failing at essentially zero changes (which is probably due rounding errors).

[dhomeier]

Dunno where codecov is still digging up 4 lines in a completely unmodified file... I'd not stopping publish however, so not that critical.
I've also tested a setup running codestyle first before anything else; might add that here as well.

[dhomeier]

@pllim do you have any experience with the coverage rounding settings to make it less strict?

[pllim]

@dhomeier , alas, no... Try ask on #infrastructure channel in Astropy Slack?

[astrofrog]

Looks good if CI passes!

[dhomeier]

Codestyle is fixed now, but there is a fresh failure on the Linux pyqt515 runs for all recent PRs. No idea why the

qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.

would have popped back up – afaics all the installed xcb/xkb packages are identical.
Can you see any change between image versions 20220619.1 and
20220729.1 that might be responsible for this @astrofrog?