Fix bare exception class in only_rerun / rerun_except causing INTERNALERROR

[janmrow]

Allow only_rerun and rerun_except in @pytest.mark.flaky to accept a single exception class directly.

Before the fix passing a bare exception class name instead of a list, caused an INTERNALERROR. The value is now handled as a single filter, consistently with string arguments.

Reproduction:

@pytest.mark.flaky(reruns=2, only_rerun=ValueError)
def test_x():
    raise ValueError("ERR")

Before: INTERNALERROR: TypeError: 'type' object is not iterable.
After: 1 failed, 2 rerun.

[janmrow]

The pytest-main failures here looks that are not related to scope of this PR. As far as I investigated, they are caused by the pytest NodeId change. Which is already implemented in #363.

[icemac]

Thank you for this PR. 😃