pyproject: hack around to make editable install work

[alexfikl]

Switching to pyproject.toml completely with setuptools seems to have broken editable installs a bit for mypy (and possibly others): python/mypy#13392.

From reading through that, there seem to be a few options to hack around it:

1. Use the current workaround with setting package-dir. Not sure why this works, but it seems to work and is used by others too.
2. Use pip install --config-settings editable_mode=strict --editable .. This seems to work, but it's easy to forget to add options to pip install.
3. Switch to a src-layout: https://packaging.python.org/en/latest/discussions/src-layout-vs-flat-layout/. Not sure what setuptools is doing differently in that case, but it seems to pick up the typing correctly with no other changes..
4. Switch to another build system. Hatchling (https://hatch.pypa.io/latest/) is small and seems to do the right thing by default for editable installs.

I mostly went with 1 because it's very non-intrusive, but it doesn't seem documented as a workaround for editable installs, so the behavior might change..

[inducer]

Thanks! It's a wee bit gross, but I can live with it. I've put the py38 back, since we do have an opinion that may differ from the default. Deleting the line length meanwhile is saying "We would like to agree with whatever the default is."

[alexfikl]

I've put the py38 back, since we do have an opinion that may differ from the default. Deleting the line length meanwhile is saying "We would like to agree with whatever the default is."

According to their docs, the default is "whatever project.requires-python" says, which seemed reasonable. Is there any case where we'd want those to disagree?
https://docs.astral.sh/ruff/settings/#target-version

[inducer]

Ah! Didn't know it was that smart. Thanks for setting me straight. c68a286