Create `matrix-python-common` and pull out some common code from Sy* projects.

[reivilibre]

The module cookiecutter template might actually be a good starting point; though you'd want to cut out the Synapse module-related fluff (but keep the CI, linting, etc?).

[H-Shay]

Noting common function _reject_invalid_json in both synapse and sygnal

[DMRobertson]

I suggest we raise an issue for each thing we identify as being duplicated.

[reivilibre]

Going to close this because the momentum has started and we're now at the point of raising individual issues for things we want to see done — thanks!