tests: Patch `os.path.exists` and improve patching for `open`. You can specify additional modules for which to patch `open`. Also, it was incorrectly updating the opened file, even when it was opened for read only. Now it only updates the contents if the file was opened for write, and supports appending to the end. Last, it supports opening text files in binary mode.

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

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

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