Change return types of AsyncGenerator's methods to coroutines. This brings pytype's definition in line with the definition on typeshed and reflects the fact that asynchronous generators defined using the `async def`/`yield` syntax have methods returning coroutines rather than just arbitrary awaitables.

[copybara-service[bot]]

Change return types of AsyncGenerator's methods to coroutines. This brings pytype's definition in line with the definition on typeshed and reflects the fact that asynchronous generators defined using the async def/yield syntax have methods returning coroutines rather than just arbitrary awaitables.