Incompatibility with xdoctest

[jayvdb]

INTERNALERROR>   File "/usr/lib/python3.8/site-packages/pluggy/callers.py", line 187, in _multicall
INTERNALERROR>     res = hook_impl.function(*args)
INTERNALERROR>   File "/usr/lib/python3.8/site-packages/pytest_matrix/plugin.py", line 38, in pytest_itemcollected
INTERNALERROR>     if isinstance(item.cls, MatrixTestBase) and item.name in item.cls.SKIP_TESTS:
INTERNALERROR> AttributeError: 'XDoctestItem' object has no attribute 'cls'