Fix mock patchings detection when both mock and unittest.mock are present

[mbachry]

If pypi mock package is installed and imported at any time during test run, it's impossible to use Python 3 @unittest.mock.patch(...) decorator. Mock object argument is not recognized by pytest with error like this:

      @unittest.mock.patch("os.path.abspath")
      def test_hello(self, abspath):
E       fixture 'abspath' not found
>       available fixtures: cache, capfd, capfdbinary, caplog, capsys, capsysbinary, doctest_namespace, monkeypatch, pytestconfig, record_xml_attribute, record_xml_property, recwarn, tmpdir, tmpdir_factory
>       use 'pytest --fixtures [testpath]' for help on them.

It's often not possible to go with unittest.mock only, as pypi mock can be implicitly pulled by one of project dependencies. In my case, it's moto package.

[coveralls]

Coverage increased (+0.0008%) to 92.633% when pulling b6166dc on mbachry:fix-unittest-mock into e7bcc85 on pytest-dev:master.

[nicoddemus]

Thanks a lot @mbachry!