[3.6] bpo-23890: Fix ref cycles in TestCase.assertRaises()

[vstinner]

bpo-23890: unittest.TestCase.assertRaises() now manually breaks a
reference cycle to not keep objects alive longer than expected.
(cherry picked from commit bbd3cf8)

[PamelaM]

Given the new try/finally handling and the potential for unexpected behavior, I think the context manager mode needs to be checked as well. I don't see a need for a separate test function, just add something like:

with self.assertRaises(ValueError):
    func()

self.assertEqual(refcount, sys.getrefcount(func))

[vstinner]

I don't see any reference cycle when using the context manager protocol.

It seems like bpo-23890 was very specific to the function API: assertRaises(ValueError, func) since you pass func to assertRaises().

with self.assertRaises(ValueError):
    func()

Here func variable may also leak in the traceback objects linked to the raisen exception. But again, I'm unable to see any leak here without my fix.

If you still consider that an unit test is needed, please comment http://bugs.python.org/issue23890