Annotations for `__eq__`

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.

Start by reading the typing specification and the linked discuss.python.org thread to understand the existing treatment of eq. The work is complete when the specification gives agreed guidance for non-overlapping comparisons, non-boolean returns, NotImplemented, and errors, with library authors able to apply it consistently.