return *(PY_LONG_LONG*)&fval == 0;
There is no aliasing, because there are no pointer variables in existence.
If we did this:

double *pfval = &fval;
PY_LONG_LONG *pl = (PY_LONG_LONG*)pfval
return *pfval == 0

Then we would have aliasing.  Because "pfval" in this example doesn't exist but is merely a temporary, there is no aliasing.

As for IEEE compatibility, I don't think we could have our own floating point formatting library if we didn't make that assumption, but I might be wrong about that.  On the other hand, I don't think there is a supported python architecture that defines positive zero as anything else than bitwise zero.  And should such a platform be found, it is easy enough to disable this code for it.