Update build system to flit_core

[takluyver]

Hi, I'm opening this pull request as part of a push to modernise how packages use Flit as a Python build backend.

Using flit_core as the backend in place of flit is recommended in the docs, and will make it faster for tools like pip & build to build your package from source, as it has fewer dependencies to install.

Specifying an explicit version range (>=2,<4) helps to ensure that your package can still be readily built from source despite changes in future major versions of Flit, because it will still use version 3.x. For instance, a future version is likely to drop support for the [tool.flit.metadata] table, in favour of the now-standardised [project] table for metadata. This is also in the docs, along with details of which versions support which features.

Most users probably install your package from a pre-built 'wheel' on PyPI, so this changes won't affect them at all. But people who install from a git checkout, for instance, will benefit.

[RMeli]

Hi @takluyver, thank you for your PR, it's really appreciated!

I haven't used flit in a long while, mainly because the CI action

spyrmsd/.github/workflows/pypi.yml

Lines 23 to 30 in d50f203

	- name: Build and publish
	env:
	FLIT_USERNAME: ${{ secrets.FLIT_USERNAME }}
	FLIT_PASSWORD: ${{ secrets.FLIT_PASSWORD }}
	run: |
	python -m pip install . --no-deps
	flit build
	flit publish

stopped working, and because the software is somewhat stable and I haven't done any release.

In #54 I tracked the issues I had with flit (that I also reported in pypa/flit#415), but manually specifying the version seemed to work (#54 (comment)). It looks like the problem was with versioneer, which I have finally removed in #91.

Two things before merging:

• Does the CI action to automatically publish the package to PyPI when a release is made need to change?
• Can you please add a line (and your GitHub handle) to the CHANGELOG?

[RMeli]

• Please also add yourself to the CITATIONS.cff file too, if you want

[takluyver]

Thanks! And sorry Flit didn't work smoothly for you.

This change doesn't need a change to your CI action, but PyPI now requires two-factor authentication, so uploading with a username & password doesn't work any longer (unless you've already configured a token in place of the 'password' - naturally I can't see this). There's a new & fancy way to automate releasing from Github, though: you can tell PyPI to trust this Github repo, and then use the pypi-publish action on Github. This handles authenticating to PyPI, so you no longer need to configure a secret manually.

[takluyver]

I've added this to the changelog. I'd feel like a fraud adding myself to the citation for a tiny mechanistic change (I don't even know what 'symmetry-corrected RMSD' means 😉 ), so I'll leave that.

[RMeli]

Thank you @takluyver!

[RMeli]

And sorry Flit didn't work smoothly for you.

Looks like removing versioner allowed to successfully deploy again using flit!

[takluyver]

Great! 😃