--run-parallel doesn't work well with old style @pytest.mark.xfail("expr", ...)

[colesbury]

Like #13, but with @pytest.mark.xfail with a string condition.

I don't have a minimal reliable reproducer, but I encountered this when running the following pybind11 tests with --run-parallel:

https://github.com/pybind/pybind11/blob/master/tests/test_unnamed_namespace_a.py

module 'pybind11_tests.buffers' has no attribute 'PYBIND11_INTERNALS_VERSION'

During handling of the above exception, another exception occurred:
Error evaluating 'xfail' condition
    (m.PYBIND11_INTERNALS_VERSION <= 4 and (m.defined___clang__ or not m.defined___GLIBCXX__)) or (m.PYBIND11_INTERNALS_VERSION >= 5 and not m.defined_WIN32_or__WIN32 and (m.defined___clang__ or m.defined__LIBCPP_VERSION))
AttributeError: module 'pybind11_tests.buffers' has no attribute 'PYBIND11_INTERNALS_VERSION'

...

ERROR tests/test_unnamed_namespace_a.py::test_have_both_class_any_struct - Failed: Error evaluating 'xfail' condition