_Breaking changes_
* Migrate from [webapp2](https://github.com/GoogleCloudPlatform/webapp2/) to [Flask](https://flask.palletsprojects.com/). webapp2 had a good run, but it's no longer actively developed, and Flask is one of the most widely adopted standalone web framework in the Python community.
* Remove `to()` class methods. Instead, now pass redirect paths to Flask's `as_view()` function, eg:
py
app = Flask()
app.add_url_rule('/start', view_func=twitter.Callback.as_view('start', '/oauth_callback'))
* Remove deprecated `blogger_v2` module alias.
* `webutil`: migrate webapp2 HTTP request handlers in the `handlers` module - `XrdOrJrdHandler`, `HostMetaHandler`, and `HostMetaXrdsHandler` - to Flask views in a new `flask_util` module.
Non-breaking changes
* `webutil`: implement [Webmention](https://webmention.net/) protocol in new `webmention` module.
* `webutil`: add misc Flask utilities and helpers in new `flask_util` module.