Define Python Raw Mem helpers in ModuleSetupCode

To handle the Python version differences, handle the definitions of
`PyMem_Raw*` functions in `ModuleSetupCode`. Then extern them in
`cpython.mem` without using a version check or a specific header.

Cython/Includes/cpython/mem.pxd

 from cpython.version cimport PY_VERSION_HEX
 
+cdef extern from *:
+    void* PyMem_RawMalloc(size_t n) nogil
+    void* PyMem_RawRealloc(void *p, size_t n) nogil
+    void PyMem_RawFree(void *p) nogil
+
 cdef extern from "Python.h":
 
     #####################################################################
@@ -24,15 +29,6 @@ cdef extern from "Python.h":
     # for the I/O buffer escapes completely the Python memory
     # manager."
 
-    IF PY_VERSION_HEX >= 0x03040000:
-        void* PyMem_RawMalloc(size_t n) nogil
-        void* PyMem_RawRealloc(void *p, size_t n) nogil
-        void PyMem_RawFree(void *p) nogil
-    ELSE:
-        void* PyMem_RawMalloc "PyMem_Malloc" (size_t n) nogil
-        void* PyMem_RawRealloc "PyMem_Realloc" (void *p, size_t n) nogil
-        void PyMem_RawFree "PyMem_Free" (void *p) nogil
-
     # The following function sets, modeled after the ANSI C standard,
     # but specifying behavior when requesting zero bytes, are
     # available for allocating and releasing memory from the Python

Cython/Utility/ModuleSetupCode.c

@@ -441,6 +441,12 @@ class __Pyx_FakeReference {
   #define PyObject_Realloc(p)  PyMem_Realloc(p)
 #endif
 
+#if CYTHON_COMPILING_IN_CPYTHON && PY_VERSION_HEX < 0x03040000
+  #define PyMem_RawMalloc(n)           PyMem_Malloc(n)
+  #define PyMem_RawRealloc(p, n)       PyMem_Realloc(p, n)
+  #define PyMem_RawFree(p)             PyMem_Free(p)
+#endif
+
 #if CYTHON_COMPILING_IN_PYSTON
   // special C-API functions only in Pyston
   #define __Pyx_PyCode_HasFreeVars(co)  PyCode_HasFreeVars(co)