Document that TestCase.assertCountEqual() can take iterables

[jkleint]

The docstring for assertCountEqual() says "An unordered sequence comparison," but in fact it works fine with general Iterables, since the first thing it does is make a defensive copy. Since order does not matter, we are not indexing into the arguments, it is necessary to read all elements to make a comparison, and a defensive copy is generally a good practice, there does not seem to be a reason to forbid iterables. That is, it is hard to imagine a situation where a list is fine but an iterable is not. (The only potential issue is if the same iterator object is passed as both arguments, but of course that is the nature of iterators.) This is, however, a change to the method contract and the implementation would have to remain consistent in the future.

I noticed the issue when using mypy, and typeshed has updated their stubs: python/typeshed#1006

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[mention-bot]

@jkleint, thanks for your PR! By analyzing the history of the files in this pull request, we identified @benjaminp, @voidspace and @serhiy-storchaka to be potential reviewers.

[rhettinger]

The docstring wasn't well written to begin with. The proposed while technically correct makes in sound even more strange. I suggest a more thorough rewording, perhaps something like, "Asserts that two iterables have the same elements, the same number of times, without regard to order", or "Asserts that two iterables have the same elements, regardless of order and that if the same element occurs more than once, it occurs the same number of times."

[jkleint]

Went with the first suggestion; thanks.