## What it does
 Checks for the presence of unused private `typing.Protocol` definitions.

 ## Why is this bad?
 A private `typing.Protocol` that is defined but not used is likely a mistake, and should
 be removed to avoid confusion.

 ## Example
 ```python
 import typing

 class _PrivateProtocol(typing.Protocol):
     foo: int
 ```

 Use instead:
 ```python
 import typing

 class _PrivateProtocol(typing.Protocol):
     foo: int

 def func(arg: _PrivateProtocol) -> None: ...
 ```