New check: consider using `Self`

[nickdrozd]

Current problem

As of version 1.0, Mypy supports the Self type. So, I would like a checker that suggests using Self where it is possible.

Desired solution

This means checking the return annotations for class methods and seeing if any can be Self.

It's not as simple as checking whether the return annotation is the same as the class. I don't understand the details exactly, but I think the method has to return the object itself, i.e. self. In this example, the increment method is a candidate for using Self, but the copy method is not:

from dataclasses import dataclass

@dataclass
class Counter:
    count: int = 0

    def increment(self) -> Counter:  # consider-using-self
        self.count += 1
        return self

    def copy(self) -> Counter:  # not Self
        return Counter(self.count)

Additional context

PEP 673 – Self Type