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

* Fix deprecation warnings due to invalid escape sequences.

* Add changelog

Signed-off-by: Karthikeyan Singaravelan <tir.karthi@gmail.com>

* update version of black and also fix the mypy config being overridden