Fixes regex for determining charset in traceback

`_coding_re` was defined to match the magic comment as described in [PEP 0263](http://legacy.python.org/dev/peps/pep-0263/), but tbtools was incorrectly using `_line_re`. This results in using `codecs.lookup` on the first line of a file, which can often match an undesired codec, e.g. 'symbol'