Fix issue where no anyio backend is produced

[ascopes]

FIxes an issue where running the outer group produces an exception "RuntimeError: Backend None Unknown".

If no anyio backend is specified, then asyncio is used as the default.

[ascopes]

for ref a workaround right now is to subclass group or command and override the call dunder.

class MyGroup(asyncclick.Group):
    def __call__(self, *_: Any, **__: Any) -> None:
        asyncio.run(self._main())


@asyncclick.group(cls=MyGroup)
async def main_group() -> None:
    ...


if __name__ == "__main__":
    main_group()

[smurfix]

Closed in acc2932

[ascopes]

Thank you!