Annotations for __eq__

[JelleZijlstra]

The spec should say something about how __eq__ methods should be written and annotated.

A few considerations:

• Enable type checkers to warn on non-overlapping equality comparisons. The type annotations should ideally allow type checkers to look at a check like A() == B() and verify whether it can ever return True.
• Some level of support for __eq__ returning a non-boolean (classic example: numpy arrays)
• How to think about __eq__ methods that return NotImplemented on unrecognized types versus those that throw an error

[JelleZijlstra]

python/typing#1536