Implement PEP 688

[JelleZijlstra]

PEP 688 has been accepted and implemented in Python 3.12. For Python 3.12 and up, this provides a clean, protocol-based way to implement buffers.

Unfortunately, we still can't directly represent buffer types on earlier Python versions. I'd like to propose the following approach:

• We pretend that typing_extensions.Buffer is a Protocol with a __buffer__ method even before 3.12.
• We create __buffer__ methods on buffer classes without a version condition, even though the method does not actually exist before 3.12.
• Now, we can define protocols that combine __buffer__ with other methods, such as the current _typeshed.SliceableBuffer.

The downside is that we're lying about the existence of the __buffer__ method, so that type checkers will not catch places where users call the method on Python < 3.12. However, hopefully this will be rare, and it is outweighed by the benefit that we can now talk about arbitrary buffer types, including third-party ones, without the big unions we're currently using in _typeshed.

[srittau]

Sounds like a good plan. While I usually don't like lying in the stubs, this is only temporary and (most likely) an overall improvement.