build: drop python 3.9 and pyside2, support python 3.14

[tlambert03]

cc @jni and @Czaki

this doesn't explicitly break pyside2, but we won't be testing it going forward

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.03%. Comparing base (c5b02dc) to head (21c4b6f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #332      +/-   ##
==========================================
+ Coverage   86.37%   87.03%   +0.65%     
==========================================
  Files          49       49              
  Lines        3869     3864       -5     
==========================================
+ Hits         3342     3363      +21     
+ Misses        527      501      -26     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[Czaki]

We dropped pyside2 in napari too. The only problem might be a person installing older napari release, but we provide constraints files from a log time so maybe we need more documentation only.