Add Python 3.14 support

[VeckoTheGecko]

Another year, another Python version - Python 3.14 is out!

We should add support for Python 3.14 to Parcels v4, and consider it for Parcels v3. I'm not sure how simple an upgrade it will be (or what caveats there are) since free threaded Python is out of experimental mode and there has been a bunch of noise in the Scientific Python community about supporting free-threaded Python. What I think might be the case is the following:

• Support Python 3.14 (non-free threaded) in v3 and v4
• (down the line) support free threaded python in v4 (good to make a separate issue)

For now all we need to do is expand our test matrices on main and v4-dev and make sure checks pass.

[erikvansebille]

Good idea! Now with pixie, it should be easy to also test Python 3.14 on v4-dev?

[VeckoTheGecko]

• (down the line) support free threaded python in v4 (good to make a separate issue)

Looking at https://py-free-threading.github.io/ I think that we'd need our dependencies to support free threading, which I don't would be happening soon.

[VeckoTheGecko]

it should be easy to also test Python 3.14 on v4-dev?

Yes, quite easy. Submitting PRs now :)