[3.12] gh-111724: Fix doctest `ResourceWarning` in `howto/descriptor.rst` (GH-111725) (#111727)

gh-111724: Fix doctest `ResourceWarning` in `howto/descriptor.rst` (GH-111725)

Close database connection explicitly in test cleanup.
(cherry picked from commit f48e669504ce53040a04e0181064c11741a87817)

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>