BLD: add option to build future-compatible abi3 wheels

[neutrinoceros]

This allows targeting CPython's LimitedAPI (and stable ABI) by setting PYPROJ_LIMITED_API=1 at build time. Resulting wheels are future compatible with yet-unreleased Python versions.
The trade-off is that runtime performance might be impacted, though I suggest measuring it against real load-bearing applications, as I found that in many numpy-based extensions the impact to be negligible or even non-measurable.
I could successfully build the package and run the test suite with this locally.

I suggest a couple possible approaches to leverage this:

• append a cp314-abi3 target to the existing build matrix. Version-specific wheels will still be prefered by installers when available but will default to these otherwise, instead of having to, and often failing to build pyproj from source.
• replace version-specific wheels with a single cp311-abi3 wheel per platform. If there are no visible performance regressions this is definitely the way to go.

[neutrinoceros]

note this comment is going to age like milk because there's a new version of the Limited API coming up in Python 3.15 that's compatible with both GIL-enabled and free-threaded builds. However there are a couple other blockers right now (Cython and setuptools both have work-in-progress patches to support this). Note that I'd be happy to revisit this later this year to add cp315-abi3.abi3t as a target

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.81%. Comparing base (5900c5a) to head (0482753).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1600      +/-   ##
==========================================
- Coverage   95.84%   95.81%   -0.04%     
==========================================
  Files          20       20              
  Lines        1879     1886       +7     
==========================================
+ Hits         1801     1807       +6     
- Misses         78       79       +1     

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[snowman2]

Mind enabling this in the wheel builds to test?

[neutrinoceros]

enabling this in CI revealed that it is currently blocked by cython

pyproj/_geod.c:1412:6: This module uses CPython specific internals of 'array.array', which are not available in PyPy or the limited API

and indeed, compilation then predictibly fails because the generated module isn't limited-api-compliant

[snowman2]

You could test it by adding a flag to use the slower alternative:

pyproj/pyproj/_compat.pyx

Line 20 in 40d7cf8

IF CTE_PYTHON_IMPLEMENTATION == "CPython":

[neutrinoceros]

thanks for the pointer. I now expect the current CI run to succeed. Should I drop the second commit before undrafting ?