remove Py_UNICODE from standard imports as it is now a builtin type

Cython/Includes/cpython/long.pxd

-from cpython.unicode cimport Py_UNICODE
 
 cdef extern from "Python.h":
     ctypedef long long PY_LONG_LONG

Cython/Includes/cpython/unicode.pxd

 cdef extern from *:
-    ctypedef unsigned int Py_UNICODE
-
     # Return true if the object o is a Unicode object or an instance
     # of a Unicode subtype. Changed in version 2.2: Allowed subtypes
     # to be accepted.