Add a pyproject.toml and some guidance around backwards compatibility

[rgommers]

A project should have a pyproject.toml nowadays. I noticed it didn't after investigating a regression reported in conda-forge and seeing that the PR for 2.1.2 didn't pick up the numpy>=2.1 version constraint in conda-forge/array-api-strict-feedstock#12. I think it would (through Grayskull) if there would be a pyproject.toml to analyze.

Related: 2.1.2, as a bugfix version, should not contain a major breaking change like that. We don't need an extensive policy I think, but a few notes in the README about what can be expected for major, minor and bugfix versions would be useful.

[asmeurer]

I think grayskull would work on setup.py metadata too.

Regarding the version number, I didn't expect this to be that breaking. But since it evidently is, we should just revert it. As I noted elsewhere, it was just a maintenance thing. We can continue to live with the extra technical debt in this repo for a little while longer.

[rgommers]

Regarding the version number, I didn't expect this to be that breaking. But since it evidently is, we should just revert it. As I noted elsewhere, it was just a maintenance thing.

Sounds good, let's do that first. Could you perhaps tag a new version and roll it out, then the blast radius will be small/short?

I think grayskull would work on setup.py metadata too.

Yeah, I didn't realize it was disabled on the feedstock - will check that.