The lists of source directories to lint between `tox.ini` and `lint.sh` became out of sync. This PR tightens them up and adds some comments reminding any future readers to keep the list in sync.

Cached functions accept an `on_invalidate` function, which we failed to add to the type signature. It's rarely used in the files that we have typed, which is why we haven't noticed it before.

This modifies the configuration of structured logging to be usable from
the standard Python logging configuration.

This also separates the formatting of logs from the transport allowing
JSON logs to files or standard logs to sockets.

This PR makes several changes to the `./scripts-dev/lint.sh` script, which lints the codebase with a number of tools:

* Adds usage information, with `-h` flag to show it. Otherwise it will show when providing an unknown flag.
* Adds option `-d` which will check both staged and unstaged files that have changed since the last commit and add them to the list of files to lint.
  - Note that only files without an extension, or with a `.py` extension will be allowed. This prevents editing bash scripts causing the linters to break on non-python files.
* Improves the print-out of which files/directories are being linted. 

https://github.com/matrix-org/synapse/tree/develop/docs/sphinx doesn't seem to really be utilised or changed recently since the initial commit. I like the idea of exportable documentation of the codebase, but at the moment after running through the build instructions the generated website wasn't very useful...

This PR adds a script that:

* Builds the local Synapse checkout using our existing `docker/Dockerfile` image.
* Downloads [Complement](https://github.com/matrix-org/complement/)'s source code.
* Builds the [Synapse.Dockerfile](https://github.com/matrix-org/complement/blob/master/dockerfiles/Synapse.Dockerfile) using the above dockerfile as a base.
* Builds and runs Complement against it.

This set up differs slightly from [that of the dendrite repo](https://github.com/matrix-org/dendrite/blob/master/build/scripts/complement.sh) (`complement.sh`, `Complement.Dockerfile`), which instead stores a separate, but slightly modified, dockerfile in Dendrite's repo rather than running the one stored in Complement's repo. That synapse equivalent to that dockerfile (`Synapse.Dockerfile`) in Complement's repo is just based on top of `matrixdotorg/synapse:latest`, which we opt to build here locally.

Thus copying over the files from Complement's repo wouldn't change any functionality, and would result in two instances of the same files. So just using the dockerfile in Complement's repo was decided upon instead.

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

Generated with:

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

This requires adding a mypy plugin to fiddle with the type signatures a bit.

Now that the server supports streaming back JSON responses, it would be nice to
show the response as it is streamed, in the test tool.

This PR:

* Reduces the amount of noise in the `check-newsfragment` CI output by hiding the dependency installation output by default.
* Prints a link to the changelog/debian changelog section of the contributing guide if an error is found.

If there are *no* files with CRLF line endings, then the xargs exits with a
non-zero exit code (as expected), but then, since that is the last thing to
happen in the script, the script as a whole exits non-zero, making the whole
thing fail.

using `if/then/fi` instead of `&& (...)` means that the script exits with a
zero exit code.

This PRs adds a script to check for unix-line terminators in the repo. It will be used to address https://github.com/matrix-org/synapse/issues/7943 by adding the check to CI.

I've changed the original script slightly as proposed in https://github.com/matrix-org/pipelines/pull/81#discussion_r460580664

Run `isort`, `flake8` and `black` over the `contrib/` directory and `synctl` script. The latter was already being done in CI, but now the linting script does it too.

Fixes https://github.com/matrix-org/synapse/issues/7910

The CI appears to use the latest version of isort, which is a problem when isort gets a major version bump. Rather than try to pin the version, I've done the necessary to make isort5 happy with synapse.

this is no longer needed on python 3

Ensure good comprehension hygiene using flake8-comprehensions.

* Raise an exception if there are pending background updates

So we return with a non-0 code

* Changelog

* Port synapse_port_db to async/await

* Port update_database to async/await

* Add version string to mocked homeservers

* Remove unused imports

* Convert overseen bits to async/await

* Fixup logging contexts

* Fix imports

* Add a way to print an error without raising an exception

* Incorporate review

This encapsulates config for a given database and is the way to get new
connections.