All development is done via GitHub using issues and pull requests. If you are doing something major (new feature, backwards incompatible change, reworking lots of code) please open an issue and discuss it before starting. If you are doing something minor (docs update, small bug fix, etc) feel free to just dive in and send a pull request.
All code should pass pep8 and pyflakes. A simple tool for checking is
flake8. You should be able to run:
flake8 .
At the root of the repository and have 0 errors or warnings. That said, everyone slips now and then, and one or two isn't a big deal and I'll just fix them myself. But if your whole patch fails then I'll likely reject it until you clean up your code.
You can report issues and feature via the issue tracker. If you have an issue please give full steps to reproduce (such as a config file that has the problem). If you have a feature you'd like, please provide as much information as possible (such as what flags in tmux the feature uses or how exactly you'd like it to work.)