Bump PyPy runtime to v3.9 @ GHA

PyPy 3.8 has a flaky bug in the garbage collector that is not going to be fixed there since that version is EoL,
but newer version have it addressed.
Here's the problem we're seeing in the CI
```TypeError: expected some sort of stmt, but got <_ast.Load object ...>
```
Here's the Link to the explanation
Resolves pytest-dev#11771

.github/workflows/test.yml

@@ -150,7 +150,7 @@ jobs:
             tox_env: "py313"
             use_coverage: true
           - name: "ubuntu-pypy3"
-            python: "pypy-3.8"
+            python: "pypy-3.9"
             os: ubuntu-latest
             tox_env: "pypy3-xdist"