Parse unitest.mock assertions as diffs

[ikonst]

Currently unitest.mock failed assertions are not recognized as "diffs" in PyCharm. This teaches the plugin to recognize those assertions and emit the EqualsAssertionError with the actual/expected.

Additionally:
pytest-mock (a popular pytest plugin providing the mocker fixture) has an improved reporting feature which patches unittest.mock and changes the format of the reports.

To work well out of the box, we handle both the stock and "patched" outputs.

For example, this test

def test_foo():
    m = Mock()
    m.foo(41)
    m.foo.assert_called_once_with(42)

would result in this output:

and diff viewer:

[ikonst]

@throwable-one can take a look?

[ikonst]

@mikekidya can review?

[ikonst]

@mikekidya ping?