🧪 Bump PyPy to v3.9

This is necessary because:
* `pypy-3.8` is EOL
* `pypy-3.8` has flaky SEGFAULTs on import [[1]] [[2]] [[3]] due to a
  bug in their GC that is fixed in `pypy-3.9`

[1]: https://github.com/jazzband/pip-tools/actions/runs/12162197242/job/33918558133?pr=2106#step:8:59
[2]: pytest-dev/pytest#11771 (comment)
[3]: https://pypy.org/posts/2024/03/fixing-bug-incremental-gc.html

.github/workflows/ci.yml

@@ -148,7 +148,7 @@ jobs:
           - MacOS
           - Windows
         python-version:
-          - pypy-3.8
+          - pypy-3.9
         pip-version:
           - latest
     env: