Add Python return type for async functions

[giarc3]

As discussed in #1970, uniffi currently doesn't put a return type on generated async functions in Python.

This is my attempt at correcting the function signatures it generates. Because the diff is a bit bad, I'll summarize the changes:

• For async functions, match on if they have a return type
  a. If they do, add async to the front of the function, add the -> ___ return type, and add await to the return statement.
  b. If they don't, still add async to the front of the function and await to the return.
• Remove the match meth.return_type() cases from the body of the function, as it's now being checked outside of the function

The changes are repeated in macros.py and TopLevelFunctionTemplate.py

b. technically could remain unchanged, but it seemed more correct to make the generated functions match each other.

Note that I'm by no means a Python expert, so please let me know if there's a more ergonomic/preferred way to accomplish all this.

[mhammond]

Thanks! This is a larger change than I was anticipating - which isn't necessarily a problem, just needs a closer look. I'll look in more detail over the next day or 2.

[mhammond]

Thanks! Please let me know if I missed something here...

[giarc3]

@mhammond I pushed a change to add the None and reduce the code duplication as you indicated (much cleaner diff!).

In doing testing of this against a local project, I happened upon the Protocol classes getting generated, and hit the extent of my knowledge. As I didn't change Protocol.py, the functions in the protocol classes remain without async and without the return type (if present). Do you know if this is an issue? I would have expected the lack of async to cause a problem, but I haven't run into anything.

[mhammond]

I think the lack of async in protocol should be OK as async is just sugar - the prototype code still returns a coroutine. The lack of annotations there isn't ideal, but I don't see how it hurts. The fact our CI is green and this has tests means I think we should land this - if it does turn out we broke something then we'll fix it and add more tests :)

Thanks!