The unittest.mock `assert_called_*with*` methods take either literals, or the single `ANY` special matcher (https://docs.python.org/3/library/unittest.mock.html#any) to match any argument.

Experience suggests me that it's useful to have more flexible matchers, such as INSTANCEOF or REGEXP.

Will provide a starting pull request for this today. (Be warned: it's the first time I try contributing to CPython.)