Add pypy back to github actions test matrix.

Author: Ryan Whitmore

.github/workflows/tests.yml

@@ -60,6 +60,7 @@ jobs:
           - "3.11"
           - "3.12"
           - "3.13"
+          - "pypy3"
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5