Allow coroutines in service methods

[michallowasrzechonek-silvair]

fixes #24

[acrisci]

Run make format to run yapf on the source files.

[michallowasrzechonek-silvair]

If this approach looks good, I think it should also work for asynchronous properties, by replacing getattr and setattr with similar callback-based setters, cf.

• https://github.com/SilvairGit/python-dbus-next/blob/async_service_methods/dbus_next/message_bus.py#L717-L719
• https://github.com/SilvairGit/python-dbus-next/blob/async_service_methods/dbus_next/message_bus.py#L728-L729.

[acrisci]

Yeah this approach seems good. That should work for properties, as long as it doesn't break the existing user interface because I like how that works right now.

[michallowasrzechonek-silvair]

Alright, I'll try to come up with something that doesn't change the API. That would be a separate PR, though.

[acrisci]

Looks good thanks 👍