[3.12] gh-120590: Fix test_pydoc in the refleak hunting mode (GH-120615) (GH-120670)

Mocking only works if sys.modules['pydoc'] and pydoc are the same,
but some pydoc functions reload the module and change sys.modules.
Ensure that sys.modules['pydoc'] is always restored after the corresponding
tests.
(cherry picked from commit 2cf47389e26cb591342d07dad98619916d5a1b15)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>