isfunction() processes both `isfunction(func)` and `isfunction(partial(func, arg))` correctly.

But `iscoroutinefunction()` misses this functionality.

We can implement it easy by adding a check for `isinstance(func, partial)` and applying a coroutine check for `func.func`.

Also, we can do the same for `isgeneratorfunction()` and `isasyncgenfunction()`.

The patch looks easy and straightforward.

Yuri, what do you think about?