https://docs.python.org/3/whatsnew/3.11.html says to use
PyFrame_GetBack() instead of direct ->f_back access which indeed starts
to create frames on the fly in py3.11 if ->f_back was NULL:

https://github.com/python/cpython/commit/ae0a2b756255
https://github.com/python/cpython/commit/68f5fa668343

-> change all usage of ->f_back to that function not to skip
yet-unmaterialized frames while doing our checks.

/reviewed-by @jerome
/reviewed-on !37