[3.11] gh-111724: Fix doctest ResourceWarning in howto/descriptor.rst (GH-111725) (#111728)

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

Close database connection explicitly in test cleanup.
(cherry picked from commit f48e669)

Co-authored-by: Nikita Sobolev <[email protected]>

Author: miss-islington

Doc/howto/descriptor.rst

@@ -943,6 +943,10 @@ it can be updated:
     >>> Movie('Star Wars').director
     'J.J. Abrams'
 
+.. testcleanup::
+
+   conn.close()
+
 
 Pure Python Equivalents
 ^^^^^^^^^^^^^^^^^^^^^^^