Revert __eq__ removal

[sobolevn]

Because __eq__ is treated specially by mypy when strict_equality is turned on.
Without __eq__ on a type, mypy raises this error:

class A:
    def __eq__(self, o: object) -> bool: ...

class B: ...

if A() == 1:  # ok
    ...

if B() == 1: # error: Non-overlapping equality check (left operand type: "B", right operand type: "Literal[1]")
    ...

Refs #8483

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[hauntsaninja]

Thanks for catching this!