I don't think we need to support prefixes without quotes or with triple qoutes. 'ur' is not valid prefix. Using simplified code from tokenize:

        _strprefixes = [''.join(u) + q
                        for t in ('b', 'r', 'u', 'f', 'br', 'rb', 'fr', 'rf')
                        for u in itertools.product(*[(c, c.upper()) for c in t])
                        for q in ("'", '"')]

Or you can use tokenize._all_string_prefixes() directly:

        _strprefixes = [p + q
                        for p in tokenize._all_string_prefixes()
                        for q in ("'", '"')]

But it may be simple to just convert the string to lower case before looking up in the symbols dict. Then

        _strprefixes = [p + q
                        for p in ('b', 'r', 'u', 'f', 'br', 'rb', 'fr', 'rf')
                        for q in ("'", '"')]