Switch build-backend to flit_core + use License-Expression for project metadata

[cdce8p]

Setuptools v77 added support for SPDX license expressions with the "new" syntax for project.license. Unfortunately, the setuptools version no longer supports Python 3.8, so we would need to drop it here as well if we want to use the new syntax.
Switch build-backend to flit-core for PEP 639 license expression support.

Metadata diff

 ...
-License: MIT
+License-Expression: MIT
 ...
-Classifier: License :: OSI Approved :: MIT License
 Classifier: Programming Language :: Python :: 3
-Classifier: Programming Language :: Python :: 3.8
 ...

This is a followup to #53.

[hauntsaninja]

Maybe we could switch to flit_core or something?

(Could also be nice to avoid forcing very recent lower bounds on setuptools given their approach towards backcompat and PIP_CONSTRAINT is not a flexible hammer)

[cdce8p]

(Could also be nice to avoid forcing very recent lower bounds on setuptools given their approach towards backcompat ...)

Flit_core also added basic support for it only recently with 3.11.

Maybe we could switch to flit_core or something?

The switch was actually quite seamless. At least for such a simple project. Nice to know that the ecosystem has finally reached this point it's working towards for so long.