Change C includes of "longintrepr.h" to account for upstream changes (GH-4428)

See https://github.com/python/cpython/pull/28968/8e5de40f90476249e9a2e5ef135143b5c6a0b512
which is part of implementing
https://bugs.python.org/issue35134
moved the header "longintrepr.h" into a sub-folder.
The notes on this change suggested to include "Python.h" instead.

Cython/Includes/cpython/longintrepr.pxd

 # Internals of the "long" type (Python 2) or "int" type (Python 3).
-# This is not part of Python's published API.
 
-cdef extern from "longintrepr.h":
+cdef extern from "Python.h":
+    """
+    #if PY_MAJOR_VERSION < 3
+     #include "longintrepr.h"
+    #endif
+    """
     ctypedef unsigned int digit
     ctypedef int sdigit  # Python >= 2.7 only
 

Cython/Utility/ModuleSetupCode.c

@@ -263,7 +263,9 @@
 #define CYTHON_BACKPORT_VECTORCALL (CYTHON_METH_FASTCALL && PY_VERSION_HEX < 0x030800B1)
 
 #if CYTHON_USE_PYLONG_INTERNALS
-  #include "longintrepr.h"
+  #if PY_MAJOR_VERSION < 3
+    #include "longintrepr.h"
+  #endif
   /* These short defines can easily conflict with other code */
   #undef SHIFT
   #undef BASE

tests/compile/pylong.pyx

@@ -8,7 +8,12 @@ cdef extern from "Python.h":
         Py_ssize_t ob_refcnt
         PyTypeObject *ob_type
 
-cdef extern from "longintrepr.h":
+cdef extern from "Python.h":
+    """
+    #if PY_MAJOR_VERSION < 3
+     #include "longintrepr.h"
+    #endif
+    """
     cdef struct _longobject:
         int ob_refcnt
         PyTypeObject *ob_type