PEP 660 support

[sbidoul]

This is a very rough proof of concept for build_wheel_for_editable, as described in https://discuss.python.org/t/standardising-editable-mode-installs-runtime-layout-not-hooks/4098/55 and prototyped in pypa/pip#8212.

It works by adding a .pth file in the transient wheel, and should therefore behave identically as flit install --pth-file.

[sbidoul]

Updated to match the latest PEP 660 version.

[takluyver]

Thanks @sbidoul - this looks reasonable enough to me, if you'd like to add a test or two.

[sbidoul]

Thanks for the review. I should come back to this later this month.

[sbidoul]

@takluyver this should now be ready.

I'm not sure how to test buildapi.build_editable as existing tests for buildapi rely on the filt command and I don't think we should expose the editable wheel format there.

[takluyver]

Thanks, I'll take a look. :-)

I'm not sure what you mean about tests for buildapi relying on the flit command - this one looks like it's calling buildapi directly, for instance:

https://github.com/takluyver/flit/blob/734ab12fc5bfadf7e37dc303d44fd517f7ebda1b/flit_core/flit_core/tests/test_buildapi.py#L44-L49

[takluyver]

Nice and simple, thank-you. I think if you can add a simple test to exercise the function exposed in buildapi, this should be fine.

[sbidoul]

@takluyver I somehow missed the buildapi tests in flit_core. Proof I needed holidays, I guess.

I now added the missing tests.

[sbidoul]

@takluyver FYI PEP 660 has been merged in pip and will be part of the 21.3 release in October. \o/

[takluyver]

Thanks, and congratulations on getting the PEP you worked on accepted & implemented!

[sbidoul]

Thanks, it was quite a ride, I should say :)