[Cython] callable() optimization

[Vitja Makarov]

I've noticed regression related to callable() optimization.

https://github.com/cython/cython/commit/a40112b0461eae5ab22fbdd07ae798d4a72ff523

class C:
    pass
print callable(C())

It prints True optimized version checks ((obj)->ob_type->tp_call !=
NULL) condition that is True for both class and instance.

>>> help(callable)
callable(...)
    callable(object) -> bool

    Return whether the object is callable (i.e., some kind of function).
    Note that classes are callable, as are instances with a __call__() method.

-- 
vitja.