Document the step precondition of PySlice_AdjustIndices()

Also remove trailing whitespace added by a suggestion.

Author: serhiy-storchaka

Doc/c-api/slice.rst

@@ -108,6 +108,9 @@ Slice Objects
    Out of bounds indices are clipped in a manner consistent with the handling
    of normal slices.
 
+   *step* must not be zero and must not be less than ``-PY_SSIZE_T_MAX``,
+   as guaranteed by :c:func:`PySlice_Unpack`.
+
    Return the length of the slice.  Always successful.  Doesn't call Python
    code.
 

Lib/test/test_capi/test_slice.py

@@ -64,7 +64,7 @@ def test_getindices(self):
         self.assertEqual(getindices(slice(-3, -1), 10), (7, 9, 1))
         self.assertEqual(getindices(slice(-1, -3, -1), 10), (9, 7, -1))
         self.assertEqual(getindices(slice(None, None, -2), 0), (-1, -1, -2))
-        self.assertEqual(getindices(slice(-3, -5, 1), 0), (-3, -5, 1))        
+        self.assertEqual(getindices(slice(-3, -5, 1), 0), (-3, -5, 1))
 
         # It fails without setting an exception for out of bounds indices,
         # a zero step and non-integer indices.