Annotations for `__eq__`

[JelleZijlstra]

It would be useful to have more guidance on how __eq__ methods should be 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

See also https://discuss.python.org/t/make-type-hints-for-eq-of-primitives-less-strict/34240

Covering this in the typing spec is important because it affects how library authors should annotate __eq__ methods in their libraries if they want all type checkers to understand these methods.