Define extern `PyBUF_MAX_NDIM` (GH-3811)

Ensure that Cython exposes `PyBUF_MAX_NDIM` from Python as part of
`cpython.buffer` to allow access to developers.

Cython/Includes/cpython/buffer.pxd

@@ -2,6 +2,9 @@
 
 cdef extern from "Python.h":
 
+    cdef enum:
+        PyBUF_MAX_NDIM
+
     cdef enum:
         PyBUF_SIMPLE,
         PyBUF_WRITABLE,