* Adds B00 to ignored checks.
* Fixes remaining issues.

 - Update black version to the latest
 - Run black auto formatting over the codebase
    - Run autoformatting according to [`docs/code_style.md
`](https://github.com/matrix-org/synapse/blob/80d6dc9783aa80886a133756028984dbf8920168/docs/code_style.md)
 - Update `code_style.md` docs around installing black to use the correct version

If we are lacking an optional dependency, skip the tests that rely on it.

The final part (for now) of my work to implement a username picker in synapse itself. The idea is that we allow
`UsernameMappingProvider`s to return `localpart=None`, in which case, rather than redirecting the browser
back to the client, we redirect to a username-picker resource, which allows the user to enter a username.
We *then* complete the SSO flow (including doing the client permission checks).

The static resources for the username picker itself (in 
https://github.com/matrix-org/synapse/tree/rav/username_picker/synapse/res/username_picker)
are essentially lifted wholesale from
https://github.com/matrix-org/matrix-synapse-saml-mozilla/tree/master/matrix_synapse_saml_mozilla/res. 
As the comment says, we might want to think about making them customisable, but that can be a follow-up. 

Fixes #8876.

This was never used, so let's get rid of it.

A few test cases were relying on being able to mount non-client servlets on the
test resource. it's better to give them their own Resources.

Rather than using a single JsonResource, construct a resource tree, as we do in
the prod code, and allow testcases to add extra resources by overriding
`create_resource_dict`.

These are now only available via `/_synapse/admin/v1`.

remove the stubbing out of `request.process`, so that `requestReceived` also renders the request via the appropriate resource.

Replace render() with a stub for now.

The root resource isn't necessarily a JsonResource, so rename this method
accordingly, and update a couple of test classes to use the method rather than
directly manipulating self.resource.

Add `displayname` to Shared-Secret Registration for admins to `POST /_synapse/admin/v1/register`

These are tests for #8439

Should fix #3365.

This is now redundant, and we can just call `handle_new_client_event` directly.

This converts calls like super(Foo, self) -> super().

Generated with:

    sed -i "" -Ee 's/super\([^\(]+\)/super()/g' **/*.py

This PR adds a confirmation step to resetting your user password between clicking the link in your email and your password actually being reset.

This is to better align our password reset flow with the industry standard of requiring a confirmation from the user after email validation.

This ended up being a bit more invasive than I'd hoped for (not helped by
generic_worker duplicating some of the code from homeserver), but hopefully
it's an improvement.

The idea is that, rather than storing unstructured `dict`s in the config for
the listener configurations, we instead parse it into a structured
`ListenerConfig` object.

* Factor out functions for injecting events into database

I want to add some more flexibility to the tools for injecting events into the
database, and I don't want to clutter up HomeserverTestCase with them, so let's
factor them out to a new file.

* Rework TestReplicationDataHandler

This wasn't very easy to work with: the mock wrapping was largely superfluous,
and it's useful to be able to inspect the received rows, and clear out the
received list.

* Fix AssertionErrors being thrown by EventsStream

Part of the problem was that there was an off-by-one error in the assertion,
but also the limit logic was too simple. Fix it all up and add some tests.

This mostly just reduces the amount of "running from sentinel context" spam
during unittest setup.

this is never set to anything other than "test", and is a source of unnecessary
boilerplate.

* Pull Sentinel out of LoggingContext

... and drop a few unnecessary references to it

* Factor out LoggingContext.current_context

move `current_context` and `set_context` out to top-level functions.

Mostly this means that I can more easily trace what's actually referring to
LoggingContext, but I think it's generally neater.

* move copy-to-parent into `stop`

this really just makes `start` and `stop` more symetric. It also means that it
behaves correctly if you manually `set_log_context` rather than using the
context manager.

* Replace `LoggingContext.alive` with `finished`

Turn `alive` into `finished` and make it a bit better defined.

per matrix-org/matrix-doc#2432