byteify-strings: handle triple quoted strings if they are not docstrings

As with anything in this script, this is a best effort approach. Most of the
time, when a triple quoted string is assigned to something, it's not a
docstring.

contrib/byteify-strings.py

@@ -139,6 +139,8 def replacetokens(tokens, opts):
             # components touching docstrings need to handle unicode,
             # unfortunately.
             if s[0:3] in ("'''", '"""'):
+                # If it's assigned to something, it's not a docstring
+                if not _isop(i - 1, '='):
                 yield adjusttokenpos(t, coloffset)
                 continue