Adopt SPEC 0 instead of NEP-29

[dcherian]

What is your issue?

https://docs.xarray.dev/en/stable/getting-started-guide/installing.html#minimum-dependency-versions says that we follow NEP-29, and I think our min versions script also does that.

I propose we follow https://scientific-python.org/specs/spec-0000/

In practice, I think this means we mostly drop Python versions earlier.

[keewis]

We follow something that is close to both, but not quite: our policy is to support a version at least n months, which in practice means that we can drop a version once the version we bump to is at least n months old. As long as we choose n appropriately, that means that the schedules align (but it can happen that we support a package longer than SPEC-0 would require).