Add Python 3.14 to test matrix

[jezdez]

Summary

Adds Python 3.14 testing support. Python 3.14.3 is now available on both conda-forge and defaults channels.

Changes:

• Added Programming Language :: Python :: 3.14 classifier
• Added pixi feature py314 and test-py314 environment
• Added "314" to the CI matrix in tests.yml
• Updated pixi.lock

Test plan

• All 20 quick tests pass locally on Python 3.14.3
• CI passes on all platforms with Python 3.14

[jezdez]

The py314 + defaults matrix exclusion is needed because conda-self isn't built for Python 3.14 on the defaults channel yet. A PR to enable that has been opened: AnacondaRecipes/conda-self-feedstock#2

The feedstock PR has been merged and conda-self 0.1.1 build 1 with py314 is now available on the defaults channel. The matrix exclusion has been removed.

[marcoesters]

The py314 + defaults matrix exclusion is needed because conda-self isn't built for Python 3.14 on the defaults channel yet. A PR to enable that has been opened: AnacondaRecipes/conda-self-feedstock#2

The feedstock PR has been merged and conda-self 0.1.1 build 1 with py314 is now available on the defaults channel. The matrix exclusion has been removed.

Looks like Windows has a package conflict even though conda-self is available. Maybe re-run that workflow?