Ensure all "import dns.*" statements are always placed after
pytest.importorskip('dns') calls, in order to allow the latter to
fulfill their purpose. Explicitly import all dnspython modules used by
each dnspython-based test to avoid relying on nested imports. Replace
function-scoped imports with global imports to reduce code duplication.
(cherry picked from commit 49312d6bb2)
30bbbc7aef