👽️ Ensure compatibility with AnyIO 4.11.0

[svlandeg]

Recently, our CI broke after the release of AnyIO 4.11.0.

In the syncify function implementation, we are relying on some internals of anyio:

       current_async_module = (
            getattr(threadlocals, "current_async_backend", None)
            or
            # TODO: remove when deprecating AnyIO 3.x
            getattr(threadlocals, "current_async_module", None)
        )

Then when current_async_module is not None, we decide we're in an async context.

In agronholm/anyio#973 however, these internals were changed.

As such, we should now be checking threadlocals.current_token instead.