* Remove `trial` section from setup.cfg

This was added in the initial commit from 2014. I can't see that it does
anything. Maybe it's there so that you can run `trial` without any extra
args, but if I do that then I just get the `--help` message.

* Move flake8's config to its own file

Part of #9366

Adds in fixes for B006 and B008, both relating to mutable parameter lint errors.

Signed-off-by: Jonathan de Jong <jonathan@automatia.nl>

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

The hashes are from commits due to auto-formatting, e.g. running black.

git can be configured to use this automatically by running the following:

    git config blame.ignoreRevsFile .git-blame-ignore-revs

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...

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.

Broadly three things here:

* disable W504 which seems a bit whacko
* remove a bunch of `as e` expressions from exception handlers that don't use
  them
* use `r""` for strings which include backslashes

Also, we don't use pep8 any more, so we can get rid of the duplicate config
there.

Since, for better or worse, we seem to have configured isort to generate
89-character lines, pycharm is now complaining at me that our lines are too
long.

So, let's configure pep8 to behave consistently with flake8.

Apparently flake8 v3 doesn't like trailing comments on config settings.

Also remove the pep8 config, which didn't work (because it was missing W503)
and duplicated the flake8 config. We don't use pep8 on its own, so the config
was duplicative.

pycharm supports them so there is no need to use the other format.

Might as well convert the existing strings to reduce the risk of
people accidentally cargo culting the wrong doc string format.