chore: 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.

[takluyver]

👋 I'd be very grateful if someone could take a look, because this repo serves as a template for lots of novice users, and it's leading them to do something that's likely to break in the future.

[dciborow]

👋 I'd be very grateful if someone could take a look, because this repo serves as a template for lots of novice users, and it's leading them to do something that's likely to break in the future.

so sorry for the delay. (I dont get to do as much Python as I used to)

[takluyver]

Thanks @dciborow!