Introduce ExitCode.COLLECTION_ERROR for collection errors

[SemTiOne]

Closes #4603

Previously collection errors exited 2, they now exit 7 (ExitCode.COLLECTION_ERROR).

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[RonnyPfannschmidt]

I like it

We may need to assess if this is breaking for some people

[RonnyPfannschmidt]

ci fixed needed

[Pierre-Sassoulas]

LGTM. More tests changed than code is a good sign.

[RonnyPfannschmidt]

please investigate what triggered the rtd failure

[SemTiOne]

GTG.

[bluetech]

LGTM. I left a few comments but nothing to change really.

[bluetech]

(Preexisting) I'm curious why this check happens in pytest_runtestloop instead of pytest_collection. To me it makes more sense as part of the collection phase, instead of runtest phase needing to check for "leftovers" from collection.

[bluetech]

IMO a lower level shouldn't refer to what a higher level does with it. It can easily become outdated.

Suggested change

	Subclasses ``Interrupted`` for compatibility; ``wrap_session`` maps it
	to ``ExitCode.COLLECTION_ERROR``.
	Subclasses ``Interrupted`` for compatibility.

[bluetech]

This looks a bit ugly to me, in the sense that as a user it would look to me like pytest's implementation leaks into the user interface. Interrupted is less jarring because it's a proper word. Though I guess KeyboardInterrupt is the same. So it's fine.

An alternative is to avoid a separate CollectionInterrupted exception and use a parameter to Interrupted instead. But I figure you intentionally separated the two, since "interrupted" is really not a good fit for collection errors, but have to do it for backward compat.

[bluetech]

Suggested change

	if isinstance(excinfo.value, exit.Exception):
	elif isinstance(excinfo.value, exit.Exception):

[bluetech]

(Preexisting) I wonder why we have _report_keyboardinterrupt both in pytest_sessionfinish and in pytest_unconfigure. Probably there's a good reason, I wish there was a code comment for it.