Remove nix

[qqshfox]

1. Introduce Poetry to lock dependency versions
2. Remove nix stuff
3. Add plain-Dockerfile

Resolves #288

[brettdh]

I'm not a maintainer, but I suspect this would be more likely to be accepted if it adds a plain Dockerfile rather without removing the Nix setup. This could include removing the dockerize stuff from the Nix setup, but not removing the Nix setup altogether. As was discussed on #288, there are still users who prefer to deploy marge-bot via Nix.

[qqshfox]

@nejch thanks for the feedback. I've submitted a new MR to upgrade the dependencies and fix lint warnings introduced by the latest linters. However other parts, e.g. testing with more python versions, plain-Dockerfile, are all heavily based on the removal of nix. I'm not sure if it's viable to rewrite them along with the nix stuff.

[nejch]

@nejch thanks for the feedback. I've submitted a new MR to upgrade the dependencies and fix lint warnings introduced by the latest linters. However other parts, e.g. testing with more python versions, plain-Dockerfile, are all heavily based on the removal of nix. I'm not sure if it's viable to rewrite them along with the nix stuff.

Yeah that's basically what I had in mind for splitting up the linting issues, thanks :)