Don't break backwards compatibility

[Ryuno-Ki]

Looking at https://github.com/Ryuno-Ki/webmention-tools/compare/0.4.0..f1ddd7e8e404ba0650f0188f33c262fc772f4a6d#diff-a694b71f75a336c099dcdc529b8a376c I noticed, that right now, the backend compatibility is broken.
Meaning, the optional parameter should still be handled.
Likely other signatures have to be checked as well.

Thanks @pakal for writing API stability is cheap and easy, with Compat Patchers!

[Ryuno-Ki]

Hey @snarfed,
I have a day off today so I looked into the PRs open on this repo.
Turns out, that it fails mostly over Python2.7.

What's the timeframe for bridgy on your end to drop that version?
I'd leave this repo stalled until I can safely drop Python2.7, because patching my code for making it work looks quite hard right now.

[snarfed]

hey, thanks for asking! and sorry python 2 is holding you back.

i can't wait to migrate bridgy to python 3! sadly it's on App Engine Standard's python 2 APIs, which still don't have a complete migration path to 3: https://cloud.google.com/appengine/docs/standard/python3/python-differences#migrating_between_python_2_and_python_3_on_the_app_engine_standard_environment . they're making progress, but ndb is still in progress, and memcache doesn't exist.

feel free to go ahead and drop python 3 from master here if you want though! i can use my fork in the short term, until the App Engine stuff gets worked out.