* Runs all linters, formatters and type-checkers with pre-commit
  via tox
* Removes redundant dev dependecies from pyproject.toml
  and poetry.lock
* Removes the shell scripts we had for code checks
* Uses whichever Python version is available in the environment
* To run tools individually, you can do so with
  `poetry run pre-commit run <hook-id>`
* To update all pre-commit tools to their latest versions, run
`poetry run pre-commit autoupdate`

This patch also
* Formats all files in the repo to conform to the new standard
* Moves all dev dependecies inside pyproject.toml to
  [tool.poetry.group.dev.dependencies], as the old section
  naming is about to become deprecated

Bug: T320234
Change-Id: Ia55261334ade5d8703a568da1ee29aa5b84b7ea6