pasteurize does not refactor "yield from" statements

[rspeer]

I'm hoping to actually use pasteurize to maintain a py2 branch of a py3 codebase, langcodes.

Right now I change all the py3 syntax to py2 syntax manually in a branch of unmerged git commits, which is a pain. It would be great if pasteurize could automate this.

One issue I've run into: pasteurize doesn't know what to do with the "yield from" syntax. When it appears as a statement on its own, it can be translated straightforwardly, if a bit inefficiently. For example, the syntax

yield from parse_item(lines)

can become:

for _item in parse_item(lines):
    yield _item

It would probably be much harder to translate the expression form, result = yield from parse_item(lines). I can't think of a good way. PEP 380 has a 39-line monstrosity as the translation.