gh-142966: Make ctypes.POINTER.set_type also reset format (GH-142967)

Make the deprecated set_type method resets the format, using the
same code as in type initialization.

Implementation note: this was done in PyCPointerType_init 
after calling PyCPointerType_SetProto, but was forgotten
after in PyCPointerType_set_type_impl's call to
PyCPointerType_SetProto.
With this change, setting the format is conceptually part of
setting proto (i.e. the pointed-to type).

Co-authored-by: AN Long <aisk@users.noreply.github.com>