Strict type annotations

Add `--strict` mypy annotations

Earlier PR with basic support: #786 

This worked with with [libtmux](https://github.com/tmux-python/libtmux/pull/383/files) and [libvcs](https://github.com/vcs-python/libvcs/pull/390/files), let's bring the quality enhancement to tmuxp.   This should make future refactors easier for us by catching obvious mistakes