Clarify error message for non @runtime_checkable decorated protocols

[XuehaiPan]

Hi typing-extension team. typing-extension version 4.6.0 just came out. It has caused some of the CI failures on our side (https://github.com/metaopt/optree/actions/runs/5054617196/jobs/9069750456). We are going to fix this misusage as a bug on our side (as the new behavior will be the standard API in future Python releases). But I found the error message that was raised is not very clear.

typing_extensions/src/typing_extensions.py

Lines 703 to 704 in 356934c

	raise TypeError("Instance and class checks can only be used with"
	" @runtime protocols")

typing_extensions/src/typing_extensions.py

Lines 759 to 760 in 356934c

	# Exists for backwards compatibility.
	runtime = runtime_checkable

This PR changes deprecated @runtime to formal API @runtime_checkable in the error message.

---

The error in https://github.com/metaopt/optree/actions/runs/5054617196/jobs/9069750456 was raised by:

class MyProtocol(Protocol[T]):
    ...

class MyClass(MyProtocal[T]):
    ...

MyProtocal.register(MyClass)  # raises TypeError for `MyProtocol` is not `runtime_checkable`

Refs:

• Backport some recent Protocol fixes from 3.12 #161
• bpo-38908: Fix issue when non runtime_protocol does not raise TypeError cpython#26067
• Backport CPython PR 26067 #132
• Runtime-checkable protocol tests: Use @runtime_checkable, not @runtime #134
• fix(typing): add @runtime_checkable decorator for CustomTreeNode protocol class metaopt/optree#56

[AlexWaygood]

LGTM! This matches the error message that CPython uses

[srittau]

Actually, could you add a CHANGELOG entry under a new # Unreleased header?