CI: set fail-fast: false on the interpreter matrix

The pypy3.11 leg was cancelled mid-install when the CPython legs failed on an
unrelated test, so the migration's key question -- whether the dependency set
installs on PyPy 3.11 -- went unanswered. A failure on one interpreter is
precisely when the other legs' results matter most.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: vthorsteinsson

.github/workflows/python-package.yml

@@ -35,6 +35,9 @@ jobs:
           --health-retries 5
 
     strategy:
+      # Don't cancel the other interpreters when one leg fails: a failure on a
+      # single version is exactly when the others' results are most informative.
+      fail-fast: false
       matrix:
         python-version: ["3.11", "3.12", "pypy3.11"]