Thank you very much for raising the question.

@patch(...) creates _patch class instance.
For decoration _patch.__call__ is used.

    def __call__(self, func):
        if isinstance(func, type):
            return self.decorate_class(func)
        return self.decorate_callable(func)

The code can be modified to

    def __call__(self, func):
        if isinstance(func, type):
            return self.decorate_class(func)
        if inspect.iscoroutinefunction(func):
            return self.decorate_async_func(func)
        return self.decorate_callable(func)

decorate_async_func can do all the same as decorate_callable with the only difference: internal
        @wraps(func)
        def patched(*args, **keywargs):
should be replaced with
        @wraps(func)
        async def patched(*args, **keywargs):
and
                return func(*args, **keywargs)
replaced with
                return await func(*args, **keywargs)

Pretty straightforward.

I did not check the code though.
I'm pretty busy up to 3.8 feature freeze to make the PR for proposed change but I love to review the existing patch.

Do want somebody to be a champion?