monkeypatch apport excepthook

[jakkdl]

This mostly just copies over code from trio, that had logic for making sure MultiErrors got printed despite apport messing with sys.__excepthook__. I'm planning to clean up the test code a bit more, and could move it into test_formatting if you want.
I added the -s to pytest in CI to help debugging, but I'll remove it again unless you like verbose tests.

Don't love the traceback.TraceBackException patch code duplication, should probably move that into a separate helper function or something. It could maybe also live outside the if statements (?).

I left the messy commit history I racked up in jakkdl#1 in case you want to see test runs where it failed without the patch.

[coveralls]

Pull Request Test Coverage Report for Build 6709099173

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 98.694%

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Totals
Change from base Build 6698232197:	0.005%
Covered Lines:	529
Relevant Lines:	536

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💛 - Coveralls

[agronholm]

Looks mostly okay, just a few nits.

[agronholm]

One more thing: please add a changelog entry.

[jakkdl]

One more thing: please add a changelog entry.

Done 👍

[agronholm]

Thanks!