Adopt SPEC 0?

[rcomer]

There is now a note on the NEP29 page stating that it has been superseded by SPEC 0.

SPEC 0 has a shorter support window for python versions, so we could drop support for python3.9 already. It also defines support windows for some of our other dependencies e.g. scipy and matplotlib.

[bjlittle]

I've already dropped support for py39 on geovista a while ago.

I felt that nep29 was a tad conservative.

But I guess you can only move as fast as your slowest dependency.