Revert "Skip running cattrs tests on PyPy (python#272)"

This reverts commit 8dfa0a5.

Author: AlexWaygood

.github/workflows/third_party.yml

@@ -312,9 +312,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        # TODO: Add pypy3.9 back to this matrix
-        # After PyYAML/PyPy issues are fixed (see #271)
-        python-version: ["3.8", "3.9", "3.10", "3.11"]
+        python-version: ["3.8", "3.9", "3.10", "3.11", "pypy3.9"]
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps: