add missing declarations for slice C-API

Cython/Includes/cpython/slice.pxd

+from cpython.ref cimport PyObject
+
+cdef extern from "Python.h":
+
+    # PyTypeObject PySlice_Type
+    #
+    # The type object for slice objects. This is the same as slice and types.SliceType
+
+    bint PySlice_Check(object ob)
+    #
+    # Return true if ob is a slice object; ob must not be NULL.
+
+    slice PySlice_New(object start, object stop, object step)
+    #
+    # Return a new slice object with the given values. The start, stop, and step
+    # parameters are used as the values of the slice object attributes of the same
+    # names. Any of the values may be NULL, in which case the None will be used
+    # for the corresponding attribute. Return NULL if the new object could not be
+    # allocated.
+
+    int PySlice_GetIndices(object slice, Py_ssize_t length,
+                           Py_ssize_t *start, Py_ssize_t *stop, Py_ssize_t *step) except? -1
+    #
+    # Retrieve the start, stop and step indices from the slice object slice,
+    # assuming a sequence of length length. Treats indices greater than length
+    # as errors.
+    #
+    # Returns 0 on success and -1 on error with no exception set (unless one
+    # of the indices was not None and failed to be converted to an integer,
+    # in which case -1 is returned with an exception set).
+    #
+    # You probably do not want to use this function.
+    #
+    # Changed in version 3.2: The parameter type for the slice parameter was
+    # PySliceObject* before.
+
+    int PySlice_GetIndicesEx(object slice, Py_ssize_t length,
+                             Py_ssize_t *start, Py_ssize_t *stop, Py_ssize_t *step,
+                             Py_ssize_t *slicelength) except -1
+    #
+    # Usable replacement for PySlice_GetIndices(). Retrieve the start, stop, and step
+    # indices from the slice object slice assuming a sequence of length length, and
+    # store the length of the slice in slicelength. Out of bounds indices are clipped
+    # in a manner consistent with the handling of normal slices.
+    #
+    # Returns 0 on success and -1 on error with exception set.
+    #
+    # Changed in version 3.2: The parameter type for the slice parameter was
+    # PySliceObject* before.