Introduce pyupgrade
- https://github.com/asottile/pyupgrade
### Todos
- [x] Introduce `pyupgrade` hook:
- [x] Issue with `await` in `f-string`:
- With Py3.6 checks as minimum: Not possible, [requires Py3.7](https://stackoverflow.com/a/43120967).
- https://github.com/asottile/pyupgrade/issues/481
- Run with `--py36-plus`, remove `await` in `f-string` changes, run hook with `--py3-plus`.
- Fixed with pyupgrade v2.20.1, v2.21.0
- [x] Apply `pyupgrade` to Python files in `silkaj` and `tests` folders
- [x] Add `pyupgrade` job based on `pre-commit`
issue