Linting the antispam module in github actions.

[Gnuxie]

It might be worth setting up some python linting at some point in the future. In Synapse we use isort to keep imports in a certain order and black for formatting.

https://github.com/matrix-org/mjolnir/pull/250#issuecomment-1072442209

We might be able to just copy something from Synapse...