complex_coerce(): add explicit PyComplex_Check() test. Previously,

complex_coerce(): add explicit PyComplex_Check() test.  Previously,
complex_coerce() would never be called with a complex argument,
because PyNumber_Coerce[Ex] doesn't bother calling the type's coercion
method if the values already have the same type.  But now, of course,
it's possible to pass an instance of a complex *subtype*, and those
must be accepted.