Implement bpo-30399: Get rid of trailing comma in the repr of BaseException

Merged Bitbucket Importer requested to merge branch/py3.7-bpo-30399 into branch/py3.7

Created originally on Bitbucket by Yannick_Jadoul (Yannick Jadoul)

Should fix test_baseexception and test_yield_from and who-knows-what-else, and perhaps break who-know-what-tests-I’ve-overlooked.

I’ve done my best with some regexes to find tests that depend on the repr of BaseException to have the extra trailing commas, but probably easier to wait and see what happens when all tests are run.

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