Add a new version of the R30 phone-home metric, which removes a false impression of retention given by the old R30 metric (#10332)

Signed-off-by: Olivier Wilkinson (reivilibre) <olivier@librepush.net>

This PR is tantamount to running:

    python3.8 -m com2ann -v 6 tests/

(com2ann requires python 3.8 to run)

I went through and removed a bunch of cruft that was lying around for compatibility with old Python versions. This PR also will now prevent Synapse from starting unless you're running Python 3.6+.

* Simplify `start_listening` callpath

* Correctly check the size of uploaded files

Part of #9744

Removes all redundant `# -*- coding: utf-8 -*-` lines from files, as python 3 automatically reads source code as utf-8 now.

`Signed-off-by: Jonathan de Jong <jonathan@automatia.nl&gt;`>

Records additional request information into the structured logs,
e.g. the requester, IP address, etc.

This bug was discovered by DINUM. We were modifying `serialized_event["content"]`, which - if you've got `USE_FROZEN_DICTS` turned on or are [using a third party rules module](https://github.com/matrix-org/synapse/blob/17cd48fe5171d50da4cb59db647b993168e7dfab/synapse/events/third_party_rules.py#L73-L76) - will raise a 500 if you try to a edit a reply to a message.

`serialized_event["content"]` could be set to the edit event's content, instead of a copy of it, which is bad as we attempt to modify it. Instead, we also end up modifying the original event's content. DINUM uses a third party rules module, which meant the event's content got frozen and thus an exception was raised.

To be clear, the problem is not that the event's content was frozen. In fact doing so helped us uncover the fact we weren't copying event content correctly.

* 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.