Add support for assert_called()

[rouge8]

This was added in Python 3.6.

I also re-ordered the wrappers to match the order in the unittest.mock docs to make it easier to see if any are missing in the future.

I'm not actually using this, but I noticed it while trying to figure out why assert_called_once() wasn't working with mocker.spy() 😂

ref: https://docs.python.org/3/library/unittest.mock.html#unittest.mock.Mock.assert_called

[coveralls]

Coverage remained the same at ?% when pulling 6886887 on rouge8:feature/assert-called into 3dc96f9 on pytest-dev:master.

[nicoddemus]

Thanks a lot @rouge8, really appreciate you taking the time to write those PRs! 👍

If you are not in a rush to use this, I will make a new release in a few weeks.

[rouge8]

I don’t actually need this at all, so no rush!

[nicoddemus]

OK thanks!