Make support for type comments optional

PEP 484 (and therefore the current typing spec) implies that type checkers must support the old "type comment" syntax. However, this syntax is unnecessary since Python 3.6 and PEP 526, so there is little reason for new type checkers to support it.

I think we should no longer make support for type comments mandatory. We can keep describing the intended semantics of type comments, but say that type checkers are free to drop support for them. Code that is intended to work across type checkers (e.g., library stubs) should not use them.

PEP 484 (and therefore this spec) imply that type checkers must support the old "type comment" syntax. However, this syntax is unnecessary since Python 3.6 and PEP 526, so there is little reason for new type checkers to support it.

I think we should no longer make support for type comments mandatory. We can keep describing the intended semantics of type comments, but say that type checkers are free to drop support for them. Code that is intended to work across type checkers (e.g., library stubs) should not use them.

(Note that this does not include # type: ignore comments. Those are still useful and should continue to work.)

This isn't urgent so I'll likely not pursue this very soon, but I'm filing this issue to keep track of the idea. I previously filed JelleZijlstra/typing-spec#1, where one person spoke up because they still like type comments, but I'm planning to archive that repo.

Read the typing specification’s treatment of type comments alongside PEP 484 and PEP 526, then review the prior JelleZijlstra/typing-spec#1 discussion. Done means the specification permits type checkers to omit ordinary type-comment support while preserving the requirement for # type: ignore comments and explaining the compatibility guidance.