Add Python 3.14 builds; bump test dependencies to match

[moreati]

This is semi-optimistic concurrency, given 3.14 is still in RC phase. The test suite ran succcessfully locally (see comment in PR). I synced uv.lock under Python 3.8 and 3.140rc2, the changes were idempotent.

I went for

• oldest viable looking Maturin (best availability to end-users)
• newest available pytest (benefit from latest fixes in CI jobs)

Maturin 1.8.7 contains a fix for Python 3.14t on Windows. Maturin 1.9.3 is newest as a I write this.

PyTest 8.4.0 was the first release with official Python 3.14 support. PyTest 8.4.1 is newest as I write this.

See

• https://www.maturin.rs/changelog.html#187
• https://docs.pytest.org/en/stable/changelog.html#pytest-8-4-1-2025-06-17

[moreati]

➜  python-libipld git:(py3.14) ✗ uv run pytest
============================================================================================ test session starts =============================================================================================
platform darwin -- Python 3.14.0rc2, pytest-8.4.1, pluggy-1.6.0
benchmark: 5.1.0 (defaults: timer=time.perf_counter disable_gc=False min_rounds=5 min_time=0.000005 max_time=1.0 calibration_precision=10 warmup=False warmup_iterations=100000)
rootdir: /Users/alex/src/python-libipld
configfile: pyproject.toml
plugins: xdist-3.8.0, benchmark-5.1.0
collected 259 items                                                                                                                                                                                          

pytests/test_cid.py ....                                                                                                                                                                               [  1%]
pytests/test_dag_cbor.py ............................................................................................................................................................................. [ 68%]
.........................................                                                                                                                                                              [ 84%]
pytests/test_decode_car.py .........                                                                                                                                                                   [ 87%]
pytests/test_decode_dag_cbor_multi.py ..........................                                                                                                                                       [ 97%]
pytests/test_multibase.py ......                                                                                                                                                                       [100%]

============================================================================================ 259 passed in 0.69s =============================================================================================

[MarshalX]

looking good! builds locally and passes tests on my side as well. idk will the CI fail or not, but gonna handle it in the next PRs. gonna merge it. thank you!