This change breaks backward compatibility in Python 2.7. This is the example that also broke in #25731. In that case the change was reverted. See https://bugs.python.org/issue25731#msg262922

$ cat foo.pxd 
cdef class B:
    cdef object b
$ cat foo.pyx 
cdef class A:
    pass

cdef class B:
    def __init__(self, b):
        self.b = b
$ cat bar.py
from foo import A, B

class C(A, B):
    def __init__(self):
        B.__init__(self, 1)

C()
$ cython foo.pyx && gcc -I/usr/include/python2.7 -Wall -shared -fPIC -o foo.so foo.c
$ python -c 'import bar'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "bar.py", line 7, in <module>
    C()
TypeError: foo.A.__new__(C) is not safe, use foo.B.__new__()