Implicit dependency on setuptools

[jaraco]

This project has an implicit dependency on setuptools. Attempting to install it from source using pip fails:

$ pip install --no-binary appdirs appdirs
Collecting appdirs
  Using cached appdirs-1.4.3.tar.gz (12 kB)
    ERROR: Command errored out with exit status 1:
     command: /usr/local/bin/python -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/private/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-install-_mxp98er/appdirs/setup.py'"'"'; __file__='"'"'/private/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-install-_mxp98er/appdirs/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /private/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-install-_mxp98er/appdirs/pip-egg-info
         cwd: /private/var/folders/c6/v7hnmq453xb6p2dbz1gqc6rr0000gn/T/pip-install-_mxp98er/appdirs/
    Complete output (3 lines):
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    ModuleNotFoundError: No module named 'setuptools'
    ----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

This project should use a pyproject.toml to declare its build dependency.

[McSinyx]

Is it time to make setuptools and wheel hard runtime dependencies for pip? Otherwise I'm pretty sure that appdirs can be built only with the standard library distutils (which pip wheel doesn't have an option to choose), hence allows it to be built on more legacy/obsolete systems. I am unaware of any pyproject.toml declaration that allow falling-back of requirements as well as build-backend however. Furthermore, IIRC PEP 518 advises that

build tools are expected to use [setuptools and wheel as build-system requirements] as their default semantics when a pyproject.toml file is not present.

More importantly, this project is nearly unmaintained (GH-79). I suggest patching downstream in case of urgency.