At 2.2, the Py<type>_Check() family of API functions (macros) changed

semantics to include subtypes.  Most concrete object APIs then had
a Py<type>_CheckExact() macro added to test for an object's type
not including subtypes.

The PyDict_CheckExact() macro wasn't created at that time, so I've added
it for API completeness/symmetry - even though nobody has complained
about its absence in the time since 2.2 was released.

Not a backport candidate.