Use serialization hooks from pytest 4.4

changelog/426.feature.rst

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+``pytest-xdist`` now uses the new ``pytest_report_to_serializable``  and ``pytest_report_from_serializable``
+from ``pytest 4.4`` (still experimental). This will make report serialization more reliable and
+extensible.
+
+This also means that ``pytest-xdist`` now also requires ``pytest>=4.4``.