Offlinemsmtp

* **Dependency change**: the `watchdog` dependency has been replaced by
`inotify`.

* Infrastructure/DX Changes

* Added pre-commit and isort
* Migrated to GitHub
* Added dependabot to auto-update GitHub Actions versions
* Converted the CI to not use Nix for linting and building (it's now way
faster)

* Require latest `PyGObject` and `watchdog` dependencies.

* Wait for one second after the path gets created on disk to allow the file to
be fully written.
* Fixed `PyGObject` dependency

* INFRASTRUCTURE

* Migrated to GitHub and GitHub Actions
* Added a custom style check for TODOs and ensuring that all instances of the
version are correct.
* Add a `shell.nix` for a more consistent development environment, and use it
with direnv.
* Got rid of `setup.py` and replaced with `pyproject.toml`.

* Use `/usr/bin/env` to find `msmtp` executable for compatibility with NixOS.

* Fixed dependency issue where sphinx was required as an `install_dependency`
rather than a dev dependency.

* INFRASTRUCTURE

* Migrated to sr.ht because of usability regressions in GitLab.
* Migrated from Pipenv to Poetry because Poetry is actually fast.
* Added `CONTRIBUTING.md` document to help onboard contributors.
* Added a `.editorconfig` file to help create consistent development
environments for contributors.

* Added the ability to to delimit arguments that should always be sent to
`msmtp` using `--`.
* Added better README documentation.
* The AUR package now automatically installs the `.service` file to
`/usr/lib/systemd/user`.
* Convert to use the logging library instead of pure print.