doctest: pytest_unconfigure: reset RUNNER_CLASS

This is important when used with ``pytester``'s ``runpytest_inprocess``.

Since 07f20ccab `pytest testing/acceptance_test.py -k test_doctest_id`
would fail, since the second run would not consider the exception to be
an instance of `doctest.DocTestFailure` anymore, since the module was
re-imported, and use another failure message then in the short test
summary info (and in the report itself):

> FAILED test_doctest_id.txt::test_doctest_id.txt - doctest.DocTestFailure: <Do...

while it should be:

> FAILED test_doctest_id.txt::test_doctest_id.txt

changelog/6039.bugfix.rst

@@ -0,0 +1,3 @@
+The ``PytestDoctestRunner`` is properly invalidated when unconfiguring the doctest plugin.
+
+This is important when used with ``pytester``'s ``runpytest_inprocess``.