Fix parsing of raw strings containing unclosed parentheses and brackets

Strings like `r'['` and `r'('` were causing the rest of the document to be
parsed as if it were inside the regex. Now we terminate the regex at the
closing quote in all cases.

This is a backport of
https://github.com/textmate/python.tmbundle/commit/2567262452b389f7042ef3026e0443f6336d1735

/cc @vmg