CI: test please_pex with Python 3.14

Python 3.14.0 was released on 2025-10-07, so we ought to start testing
the plugin against it now. Ensure that please_pex generates .pex files
that run under Python 3.14.

Author: chrisnovakovic

.github/workflows/plugin.yaml

@@ -19,6 +19,7 @@ jobs:
           - '3.11'
           - '3.12'
           - '3.13'
+          - '3.14'
   test:
     name: Test (Python ${{ matrix.python }}, stable please_pex release)
     uses: ./.github/workflows/plugin_test.yaml