Dennis, there's an existing issue on making the test suite more pypy-compatible:

https://bugs.python.org/issue25130


In that issue Maciej says pypy adopted a policy to adjust tests when they have to so I'd say that unless we get a report on which additional tests should be marked as cpython_only, I wouldn't proactively do it myself.

In fact, a RustPython core dev is specifically asking for more tests here so I don't see why we shouldn't accommodate that.