Add Python 3.14 to test matrix

Add a pixi test environment and CI matrix entry for Python 3.14,
now available on both conda-forge and defaults.

Author: jezdez

.github/workflows/tests.yml

@@ -40,7 +40,10 @@ jobs:
       matrix:
         os: [ubuntu-latest, windows-latest]
         channel: [defaults, conda-forge]
-        python-version: ["310", "311", "312", "313"]
+        python-version: ["310", "311", "312", "313", "314"]
+        exclude:
+          - channel: defaults
+            python-version: "314"
         include:
           - os: macos-15-intel
             channel: conda-forge