Added test cases for `random` argument in random.shuffle for code coverage.

Two interesting results (documented within test cases):
1.  In the docs, `random` is defined as a function returning a float in the range [0, 1), but negative floats could be returned and shuffle worked using slice notation.
2.  A dictionary with sequential numeric keys could be shuffled.

Add Mark Dickinson and Raymond Hettinger to nosy list per Brett's suggestion on core-mentorship.

* Why not use unittest.mock to test the *random* argument for shuffle?

* Try to avoid direct calls to _randbelow().  That method is a non-public implementation detail subject to change.

* Also please don't make test_random.py depend on secrets which itself depends on random.py.

I see you've added a test for the behaviour of:

    shuffle(seq, random=lambda: -1.0)

I'd suggest leaving that test out: that this works right now is really just an accident of the (CPython) implementation, and it may well not work on other Python implementations. Adding a test in effect says that there was a deliberate decision to support this (which as far as I know there isn't, and shouldn't be).

I'm also a bit uncomfortable with adding tests for other specific exceptions when a misbehaving `random` is passed: again, the details of whether an exception is raised or not for "self.assertRaises(IndexError, shuffle, seq, random=lambda: -1.1)", and which precise class of exception is raised, seem like something that could be left as implementation-defined. I'm not sure I see much value in pinning down the precise behaviour in this case (which is effectively what a testcase does).

OK, I've removed the test for the negative values returned by random.  In core-mentorship, David Murray had suggested adding a comment to document it as unexpected behavior.  I had asked there because I wasn't quite sure what to do about it.

Should I also leave out the test for being able to shuffle a dict with numeric keys?

Thanks.

I've also changed the test to use Mock for the random function.  I'm new to mock, so I'm not sure if I did it right.  Using a return_value seemed to be the best way to go.  I didn't know if it was necessary to test if an incorrect function was sent in.

New changeset f111fd2e65ef7aefd4ebeadbb48e84d609bf3733 by Raymond Hettinger (csabella) in branch 'master':
bpo-30308: Code coverage for argument in random.shuffle (#1504)
https://github.com/python/cpython/commit/f111fd2e65ef7aefd4ebeadbb48e84d609bf3733

Thank you for the patch.