bpo-47164: Add _PyASCIIObject_CAST() macro (GH-32191)

Add macros to cast objects to PyASCIIObject*, PyCompactUnicodeObject*
and PyUnicodeObject*: _PyASCIIObject_CAST(),
_PyCompactUnicodeObject_CAST() and _PyUnicodeObject_CAST(). Using
these new macros make the code more readable and check their argument
with: assert(PyUnicode_Check(op)).

Remove redundant assert(PyUnicode_Check(op)) in macros using directly
or indirectly these new CAST macros.

Replacing existing casts with these macros.