Skip running cattrs tests on PyPy (#272)

Works around #271

Author: AlexWaygood

.github/workflows/third_party.yml

@@ -312,7 +312,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.8", "3.9", "3.10", "3.11", "pypy3.9"]
+        # 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"]
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps: