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

src/_pytest/doctest.py

@@ -86,6 +86,12 @@ def pytest_addoption(parser):
     )
 
 
+def pytest_unconfigure():
+    global RUNNER_CLASS
+
+    RUNNER_CLASS = None
+
+
 def pytest_collect_file(path, parent):
     config = parent.config
     if path.ext == ".py":