[issue12029] Allow catching virtual subclasses in except clauses

[Michael McCoy]

Michael McCoy <michael.b.mccoy at gmail.com> added the comment:

Amalgamating the patch history here, I've updated the tests on Github (PR6160) to include tests for both the recursive case and ensure the correct error is propagated up if an exception occurs during the subclass check. 

I've also added a check to ensure that unittest's assertRaises behaves as expected. (The test currently passes on master, which is a bug if this doesn't get merged.)

Finally, the PR updates the documentation for try/except.

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nosy: +Michael McCoy

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