Use pypy on some test runners

[mdellweg]

Review Checklist:

• [ ] An issue is properly linked. [feature and bugfix only]
• [ ] Tests are present or not feasible.
• [ ] Commits are split in a logical way (not historically).

[mdellweg]

This was a nice experiment. But the result is that pulp cli runs significantly slower on pypy (bad usage of the JIT). Closing it for now.