Libtmux

- 321: Convert to markdown
- 271: Fix `select_window()` by providing the session ID as
argument to `-t`. Thanks Flowdalic
- Drop python 3.5 support

Python 2.7 support dropped.

- 306: chore: Remove python 2.7 support
- 314: chore: Python 3.x syntax tweaks
- 312: ci: Add tmux 3.2a to CI
- chore: Update black to [21.6b0](https://github.com/psf/black/blob/21.6b0/CHANGES.md#216b0)
- 271: Fix select_window() by providing the session ID as
argument to -t.

- 297: Enchance subprocess interaction std[in|out|err]. Needed
for interact with big buffer, fixes 251, thank you
gil-obradors!
- 303 Add `common.get_libtmux_version` which gives the tmux
version as a loose constraint. Fix linking to terms inside docs, and
duplicate description of module which sphinx warned about in api.rst.
- 266 Fix issue on local tests where env variables would cause
show-environment to pause tests indefinitely.

- 234: `Window.split_window`: Allow passing `percent`, Thank
you jinankjain!
- 289: Fix warning due to invalid escape sequences, Thank you
tirkarthi!
- 295: Publish docs via our own action
- 295: Move more packaging over to poetry, though we'll keep
setup.py for the moment to ensure compatibility package maintainers.
- 295: New development instructions
- 295: Move doc/ to docs/
- 296: CI: Test python 2.7, cache python packages, prevent running
internal PRs twice

- 278: Fix Python deprecation warnings, thanks d1618033

Also thanks Flowdalic for a similar PR at 294

- Add `project_urls` to setup.py
- 293 Move from Pipfile to poetry
- Fix show_option test in tmux 3.0
- Clean up handle_option_error comments
- Move CI to a GitHub action

- CHANGES updated to plain reStructuredText
- Add `project_urls` to setup.py for pypi.
- Looser Pipfile versions, add Pipfile.lock