Include memoryview C-API (GH-3082)

Issue: #2541

Cython/Includes/cpython/memoryview.pxd

+cdef extern from "Python.h":
+
+    ###########################################################################
+    # MemoryView Objects
+    ###########################################################################
+    # A memoryview object exposes the C level buffer interface as a Python
+    # object which can then be passed around like any other object
+
+    object PyMemoryView_FromObject(object obj)
+    # Return value: New reference.
+    # Create a memoryview object from an object that provides the buffer
+    # interface. If obj supports writable buffer exports, the memoryview object
+    # will be read/write, otherwise it may be either read-only or read/write at
+    # the discretion of the exporter.
+
+    object PyMemoryView_FromMemory(char *mem, Py_ssize_t size, int flags)
+    # Return value: New reference.
+    # Create a memoryview object using mem as the underlying buffer. flags can
+    # be one of PyBUF_READ or PyBUF_WRITE.
+    # New in version 3.3.
+
+    object PyMemoryView_FromBuffer(Py_buffer *view)
+    # Return value: New reference.
+    # Create a memoryview object wrapping the given buffer structure view. For
+    # simple byte buffers, PyMemoryView_FromMemory() is the preferred function.
+
+    object PyMemoryView_GetContiguous(object obj,
+                                      int buffertype,
+                                      char order)
+    # Return value: New reference.
+    # Create a memoryview object to a contiguous chunk of memory (in either ‘C’
+    # or ‘F’ortran order) from an object that defines the buffer interface. If
+    # memory is contiguous, the memoryview object points to the original
+    # memory. Otherwise, a copy is made and the memoryview points to a new
+    # bytes object.
+
+    bint PyMemoryView_Check(object obj)
+    # Return true if the object obj is a memoryview object. It is not currently
+    # allowed to create subclasses of memoryview.
+
+    Py_buffer *PyMemoryView_GET_BUFFER(object mview)
+    # Return a pointer to the memoryview’s private copy of the exporter’s
+    # buffer. mview must be a memoryview instance; this macro doesn’t check its
+    # type, you must do it yourself or you will risk crashes.
+
+    Py_buffer *PyMemoryView_GET_BASE(object mview)
+    # Return either a pointer to the exporting object that the memoryview is
+    # based on or NULL if the memoryview has been created by one of the
+    # functions PyMemoryView_FromMemory() or PyMemoryView_FromBuffer(). mview
+    # must be a memoryview instance.

tests/run/cython_includes.pyx

@@ -27,6 +27,7 @@ cimport cpython.long
 cimport cpython.longintrepr
 cimport cpython.mapping
 cimport cpython.mem
+cimport cpython.memoryview
 cimport cpython.method
 cimport cpython.module
 cimport cpython.number