I was a bit surprised that we hadn't double reported for popen2, but it turns
out that the implementation (at least on unix) looks like:
```
def popen2(cmd, bufsize=-1, mode='t'):
... = Popen3(cmd, False, bufsize)
...
```
but since the modeling I did only considers calls to `Popen3` only if it has
been imported from the `popen2` module, we don't consider that call as a sink.