routing: Fix breakages with routing on some versions of Python. Finally figured what happened with change 6baa94da8b16 (this is a Mercurial hash by the way if you're looking at the Git mirror). Between Python 3.6 and 3.7 there was a change where the percent sign ('%') went from being escaped by `re.escape` to _not_ being escaped. So now we need to use different regex patterns dependin on the Python version, yay.

PRIORITY_FIRST = -1
PRIORITY_NORMAL = 0
PRIORITY_LAST = 1


class Formatter(object):
    FORMAT_NAMES = None
    OUTPUT_FORMAT = None

    def __init__(self):
        self.priority = PRIORITY_NORMAL
        self.enabled = True

    def initialize(self, app):
        self.app = app

    def render(self, format_name, txt):
        raise NotImplementedError()