Add support for unicode slicing

tests/run/unicode_slicing.pyx

+# coding=utf-8
+__doc__ = u"""
+    >>> do_slice(u'abcdef', 2, 3)
+    (u'c', u'cdef', u'ab', u'abcdef')
+    >>> do_slice(u'abcdef', 0, 5)
+    (u'abcde', u'abcdef', u'', u'abcdef')
+"""
+
+import sys
+
+if sys.version_info[0] >= 3:
+    __doc__ = __doc__.replace(u"(u'", u"('").replace(u" u'", u" '")
+
+def do_slice(unicode s, int i, int j):
+    return s[i:j], s[i:], s[:i], s[:]
+