- removed version bound from `setup_requires`, allows building from
  source when only PyYaml 6.0 is available (and downloading 5.4 is
  not), #121, #125
- added type assertion to parser lookup, to make the error clearer on
  type error, #122, #123
- fixed `setup.py build_regexes` and `setup.py build` for non-in-place
  builds, #127