Oauth-dropins

_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.

Notable changes

* Add Python 3.8 support, drop 3.3 and 3.4. Python 3.5 is now the minimum required version.
* Add [Pixelfed](https://pixelfed.org/) support, heavily based on Mastodon.
* Add [Reddit](https://pixelfed.org/) support. Thanks [Will Stedden](https://bonkerfield.org/)!
* WordPress.com:
* Handle errors from access token request.

_Breaking changes_
* _Python 2 is no longer supported!_ Including the [App Engine Standard Python 2 runtime](https://cloud.google.com/appengine/docs/standard/python/). On the plus side, the [Python 3 runtimes](https://cloud.google.com/appengine/docs/standard/python3/), both [Standard](https://cloud.google.com/appengine/docs/standard/python3/) and [Flexible](https://cloud.google.com/appengine/docs/flexible/python/), are now supported.
* Replace `handlers.memcache_response()`, which used Python 2 App Engine's memcache service, with `cache_response()`, which uses local runtime memory.
* Remove the `handlers.TemplateHandler.USE_APPENGINE_WEBAPP` toggle to use Python 2 App Engine's `google.appengine.ext.webapp2.template` instead of Jinja.
* Blogger:
* Login is now based on [Google Sign-In](https://developers.google.com/identity/). The `api_from_creds()`, `creds()`, and `http()` methods have been removed. Use the remaining `api()` method to get a `BloggerClient`, or `access_token()` to make API calls manually.
* Google:
* Replace `GoogleAuth` with the new `GoogleUser` NDB model class, which [doesn't depend on the deprecated oauth2client](https://google-auth.readthedocs.io/en/latest/oauth2client-deprecation.html).
* Drop `http()` method (which returned an `httplib2.Http`).
* Mastodon:
* `StartHandler`: drop `APP_NAME`/`APP_URL` class attributes and `app_name`/`app_url` kwargs in the `to()` method and replace them with new `app_name()`/`app_url()` methods that subclasses should override, since they often depend on WSGI environment variables like `HTTP_HOST` and `SERVER_NAME` that are available during requests but not at runtime startup.
* `webutil`:
* Drop `handlers.memcache_response()` since the Python 3 runtime doesn't include memcache.
* Drop `handlers.TemplateHandler` support for `webapp2.template` via `USE_APPENGINE_WEBAPP`, since the Python 3 runtime doesn't include `webapp2` built in.
* Remove `cache` and `fail_cache_time_secs` kwargs from `util.follow_redirects()`. Caching is now built in. You can bypass the cache with `follow_redirects.__wrapped__()`. [Details.](https://cachetools.readthedocs.io/en/stable/#cachetools.cached)

Non-breaking changes
* Add Meetup support. (Thanks [Jamie Tanna](https://www.jvt.me/)!)
* Blogger, Google:
* The `state` query parameter now works!
* Add new `outer_classes` kwarg to `button_html()` for the outer `<div>`, eg as Bootstrap columns.
* Add new `image_file` kwarg to `StartHandler.button_html()`

Notable changes
* Add LinkedIn and Mastodon!
* Add Python 3.7 support, and improve overall Python 3 compatibility.
* Add new `button_html()` method to all `StartHandler` classes. Generates the same button HTML and styling as on [oauth-dropins.appspot.com](https://oauth-dropins.appspot.com/).
* Blogger: rename module from `blogger_v2` to `blogger`. The `blogger_v2` module name is still available as an alias, implemented via symlink, but is now deprecated.
* Dropbox: fix crash with unicode header value.
* Google: fix crash when user object doesn't have `name` field.
* Facebook: [upgrade Graph API version from 2.10 to 4.0.](https://developers.facebook.com/docs/graph-api/changelog)
* Update a number of dependencies.
* Switch from Python's built in `json` module to [`ujson`](https://github.com/esnme/ultrajson/) (built into App Engine) to speed up JSON parsing and encoding.

Notable changes

* _Breaking change_: switch from [Google+ Sign-In](https://developers.google.com/+/web/signin/) ([which shuts down in March](https://developers.google.com/+/api-shutdown)) to [Google Sign-In](https://developers.google.com/identity/). Notably, this removes the `googleplus` module and adds a new `google_signin` module, renames the `GooglePlusAuth` class to `GoogleAuth`, and removes its `api()` method. Otherwise, the implementation is mostly the same.
* webutil.logs: return HTTP 400 if `start_time` is before 2018-04-01 (App Engine's rough launch window).

Notable changes
* Fix dev_appserver in Cloud SDK 219 / `app-engine-python` 1.9.76 and onward. [Background.](https://issuetracker.google.com/issues/117145272#comment25)
* Upgrade `google-api-python-client` from 1.6.3 to 1.7.4 to [stop using the global HTTP Batch endpoint](https://developers.googleblog.com/2018/03/discontinuing-support-for-json-rpc-and.html).
* Other minor internal updates.