Fix tests for pytest 3

[vincentbernat]

Hey!

Many tests are failing when running with pytest 3.0.1:

_____________________________ TestMockerStub.test_failure_message_with_no_name _____________________________

self = <test_pytest_mock.TestMockerStub instance at 0x7f0b7f55c758>
mocker = <pytest_mock.MockFixture object at 0x7f0b7f51add0>

    def test_failure_message_with_no_name(self, mocker):
>       self.__test_failure_message(mocker)

test_pytest_mock.py:182:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <test_pytest_mock.TestMockerStub instance at 0x7f0b7f55c758>
mocker = <pytest_mock.MockFixture object at 0x7f0b7f51add0>, kwargs = {}, expected_name = 'mock'
expected_message = 'Expected call: mock()\nNot called'
stub = <MagicMock spec='function' id='139687357426384'>, exc_info = <ExceptionInfo AssertionError tblen=3>
@py_assert1 = AssertionError('Expected call: mock()\nNot called',)

    def __test_failure_message(self, mocker, **kwargs):
        expected_name = kwargs.get('name') or 'mock'
        expected_message = 'Expected call: {0}()\nNot called'.format(expected_name)
        stub = mocker.stub(**kwargs)
        with pytest.raises(AssertionError) as exc_info:
            stub.assert_called_with()
>       assert exc_info.value.msg == expected_message
E       AttributeError: 'exceptions.AssertionError' object has no attribute 'msg'

test_pytest_mock.py:179: AttributeError

[blueyed]

Thanks for reporting.
I've rephrased the issue: it works with pytest 3 in general.
A PR would be welcome, of course.

[vincentbernat]

I'll do a PR shortly.

[nicoddemus]

Fixed in #63