Fixed #23357 -- Added small int introspection support to MySQL backend.

In the MySQL backend, updated the can_introspect_small_integer feature
flag to True. In data_types_reverse, map FIELD_TYPE.SHORT to a
SmallIntegerField. Added test to verify introspecting SmallIntegerFields
and fixed existing tests influenced by this change.
Fixed #23349 -- Clarified details about RunPython's apps argument.
Removed an unused import.
Fixed #23329 -- Allowed inherited and m2m fields to be referenced in the admin.

Thanks to Trac alias Markush2010 and ross for the detailed reports.