Speedup str[a:b:step] for step != 1
Speedup str[a:b:step] for step != 1

Try to stop the scanner of the maximum character before the end using a limit
depending on the kind (e.g. 256 for PyUnicode_2BYTE_KIND).

sk-/python2.7-type-annotator,sk-/python2.7-type-annotator,sk-/python2.7-type-annotator