Added support for calling Python iterators.

I noticed that I couldn't call python iterators, so I spent way too much time figuring out how to.

I wasn't sure about the type conversion, but `convert(PyAny, o)` for the return object seems to work.

I found them to be pretty brittle without the `sigatomic_begin/end`, and they're not very performant, but this is why I generally stay within julia.