Add missing declarations to number.pxd (GH-3421)

PyNumber_MatrixMultiply() and PyNumber_InPlaceMatrixMultiply() were
missing from number.pxd.

The comments are taken from the C API documentation
(https://docs.python.org/3.8/c-api/number.html).

Cython/Includes/cpython/number.pxd

@@ -27,6 +27,13 @@ cdef extern from "Python.h":
     # failure. This is the equivalent of the Python expression "o1 *
     # o2".
 
+    object PyNumber_MatrixMultiply(object o1, object o2)
+    # Return value: New reference.
+    # Returns the result of matrix multiplication on o1 and o2, or
+    # NULL on failure. This is the equivalent of the Python
+    # expression "o1 @ o2".
+    # New in version 3.5.
+
     object PyNumber_Divide(object o1, object o2)
     # Return value: New reference.
     # Returns the result of dividing o1 by o2, or NULL on
@@ -133,6 +140,13 @@ cdef extern from "Python.h":
     # failure. The operation is done in-place when o1 supports
     # it. This is the equivalent of the Python statement "o1 *= o2".
 
+    object PyNumber_InPlaceMatrixMultiply(object o1, object o2)
+    # Return value: New reference.
+    # Returns the result of matrix multiplication on o1 and o2, or
+    # NULL on failure. The operation is done in-place when o1 supports
+    # it. This is the equivalent of the Python statement "o1 @= o2".
+    # New in version 3.5.
+
     object PyNumber_InPlaceDivide(object o1, object o2)
     # Return value: New reference.
     # Returns the result of dividing o1 by o2, or NULL on failure. The