tracer: fix on pypy-3.11

[rroohhh]

pypy does not have _varname_from_oparg, so fall back to pre cpython-3.11 path

ref: https://github.com/pypy/pypy/blob/0253c85bf5f86a45760eda7f0af8953ed8a253c7/lib-python/3/dis.py#L351

[rroohhh]

So now this hits pypy/pypy#3183 in sby, so blocked on YosysHQ/sby#323 I guess.

[whitequark]

Let's add a @unittest.skipIf to those sby tests; it's not like missing them on a single PyPy version is going to impact anything.

[whitequark]

Thanks!