Remove private substring() function, reuse public PyUnicode_Substring()
Remove private substring() function, reuse public PyUnicode_Substring()

 * PyUnicode_Substring() now fails if start or end is invalid
 * PyUnicode_Substring() reuses PyUnicode_Copy() for non-exact strings

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