Always bind Cython functions in Py3 (GH-3683)

Rebased 1bb26b96 for cython 0.29.x,
and made conditional to Python 3.  This does not solve the original staticmethod problem
in Python 2 but it does resolve classmethod problems in Python 3.9.  Therefore, it unbreaks
other packages on Python 3.9, improves overall state for Python 3 and does not introduce
regressions in Python 2.

Cython/Utility/CythonFunction.c

@@ -548,6 +548,7 @@ static int __Pyx_CyFunction_traverse(__pyx_CyFunctionObject *m, visitproc visit,
 
 static PyObject *__Pyx_CyFunction_descr_get(PyObject *func, PyObject *obj, PyObject *type)
 {
+#if PY_MAJOR_VERSION < 3
     __pyx_CyFunctionObject *m = (__pyx_CyFunctionObject *) func;
 
     if (m->flags & __Pyx_CYFUNCTION_STATICMETHOD) {
@@ -563,6 +564,7 @@ static PyObject *__Pyx_CyFunction_descr_get(PyObject *func, PyObject *obj, PyObj
 
     if (obj == Py_None)
         obj = NULL;
+#endif
     return __Pyx_PyMethod_New(func, obj, type);
 }