Switch to flit build backend

[blink1073]

Fixes #704

• Make sure the labextension and nbextension are properly installed from sdist and wheel

[blink1073]

@minrk, interesting limitation here. setuptools allows prereleases to satisfy version constraints, which is not technically correct. flit uses the more correct version constraint. So, we end up with a pyzmq constraint that we cannot satisfy. We could potentially install without --pre and then have a script that explicitly installs all of the direct dependencies with --pre . That script could go in maintainer-tools, similar to the minimum version handler. WDYT?

[minrk]

I don't quite understand how flit is involved in resolving prereleases. Isn't pip used for install?

I don't care too much about preleases (certainly not prerelease pyzmq which isn't used) in the build step, but I do think it's important to test against prereleases.

I also think it's just not true that allowing prereleases to satisfy constraints is incorrect. According to PEP440, inclusive ordering (>=) should accept prereleases. Prereleases are only excluded for exclusive ordering (>). Whatever code is rejecting that is not doing the right thing.

[minrk]

Looks like the bug is already reported in pip: pypa/pip#10222

[minrk]

Tested the wheel from the actions artifacts. Extensions are working as before.

Thanks!

[blink1073]

Sweet, thanks for the assist!

[minrk]

Thanks! Going to make a release next week with the various build fixes.