Use Python Limited API

[ConorMacBride]

Not sure if we want to do this. Basically, it compiles the extensions in such a way that they are compatible with all the Python versions, so we only need one wheel per OS. It will still test the wheel across all supported Python version so there won't be much of a speed up when publishing.

The ANA tests passed for me locally on macOS. Will see if all the tests pass in the CI. This is assuming the tests have good coverage of the C extension.

This is based on astrofrog/fast-histogram#56. See also PEP 384 – Defining a Stable ABI.

[nabobalis]

Is there a downside?

[Cadair]

I have no issues with this, I know of no downsides. It would be interesting to get astropy/extension-helpers#26 in before we do this though.

[ConorMacBride]

Okay. This PR adds to sunpy what the extension-helpers PR would add automatically. (I accidentally set the Py_LIMITED_API macro for cp36 instead of cp38, which I've now fixed here.)

[Cadair]

Put a comment here saying if you change this you need to change the line in _pyana.c or else we will forget.

[ConorMacBride]

I've added the comment. Here's the working CI from before the comment was added: https://github.com/sunpy/sunpy/actions/runs/2371727589

[nabobalis]

Silly question, how does one get the version hex? Is it just a conversion of the number to hexadecimal?

[ConorMacBride]

It's defined here: https://docs.python.org/3.12/c-api/apiabiversion.html

And more general details here: https://docs.python.org/3/c-api/stable.html#c.Py_LIMITED_API

[Cadair]

If we rebase this we can see the CI run?

[ConorMacBride]

Does this need a changelog?

[nabobalis]

Up to you, we could add a trivial one?