py-altair: add py314 subport

[jowens]

Description

Altair 6.0.0 declares typing-extensions>=4.12.0; python_version < '3.15', so py314-typing_extensions is still required.

Tested (just to make sure altair works with python3.14) with:

python3.14 -m venv /tmp/altair-test && /tmp/altair-test/bin/pip install altair && /tmp/altair-test/bin/python -c "import altair as alt; print(alt.__version__); c = alt.Chart({'values': [{'x': 1}]}).mark_point(); print('OK')"

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 26.4 25E246 arm64
Xcode 26.4 17E192

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@stromnov for port py-altair.