Shed

- supports Python-3.9-only files
- improved idempotence in `--refactor` mode with Teyit

- requires newer dependencies - `black` 20.8b1 or later, `autoflake` 1.4 or later

- add [`com2ann`](https://pypi.org/project/com2ann/) to `--refactor` on Python 3.8+

- use multiple processes if formatting many files (ncpus times faster!)

- drop `docformatter` due to poor performance
- reorganise remaining passes for speed and split out `--refactor` passes

- use [`pybetter`](https://pypi.org/project/pybetter/) codemods
- use [`teyit`](https://pypi.org/project/teyit/) to replace deprecated
`unittest` methods with the new aliases (if running on Python 3.9+)
- use [`docformatter`](https://pypi.org/project/docformatter/) to format docstrings
- new logic inspired by [`blacken-docs`](https://pypi.org/project/blacken-docs/)
to format code in docstrings, via the new `shed.docshed` function