Bug Report

(A clear and concise description of what the bug is.)
mypy doesn't seem to be fully a NotImplementedError in a parent class. In my parent class, I have an async def iter_x function that raises this error, and this function is actually implemented in subsequent child classes. It seems that mypy thinks the paren'ts iter function function is a coroutine because it's an async def without an await (gotten from this issue comment). As such, I get the following error: Return type "AsyncIterator[bytes]" of "iter_x" incompatible with return type "Coroutine[Any, Any, AsyncIterator[bytes]]" in supertype "X".

To Reproduce

(Write your steps here:)

1. Create an AsyncIterator async def function in a parent class that raises a NotImplementedError
2. Implement the AsyncIterator async def function in a child class. Ours does async iteration / await calls, so it has to be async def
3. Run mypy and you should get an error similar to Return type "AsyncIterator[bytes]" of "iter_x" incompatible with return type "Coroutine[Any, Any, AsyncIterator[bytes]]" in supertype "X"

Expected Behavior

(Write what you thought would happen.)
My expected behavior is that mypy would fully listen to NotImplementedError, and not make assumptions about the base method being a coroutine because of it's lack of an "await"

Actual Behavior

mypy thinks the parent class' method is a coroutine because it's an async def AsyncIterator without a yield (following this issue comment)

(Write what happened.)

Your Environment

• Mypy version used: 0.782
• Mypy command-line flags: mypy
• Mypy configuration options from mypy.ini (and other config files):
• Python version used: 3.9
• Operating system and version: Mac OS 11.4