Adapt Sentinel implementation to accepted version of PEP 661

[JelleZijlstra]

PEP-661 was accepted, but it is implemented a little differently than our typing_extensions.Sentinel. Let's see how we can make the typing-extensions version better match the version that will be in Python 3.15, while keeping backward compatibility for users who may already be using typing_extensions.Sentinel.

Here are the relevant differences:

• We have typing_extensions.Sentinel; 3.15 will have builtins.sentinel (lowercase). Let's not worry about the name. On Python 3.15 we'll re-export builtins.sentinel as typing_extensions.Sentinel.
• Our Sentinel is subclassable; builtins.sentinel will not be. Let's add an __init_subclass__ that emits a DeprecationWarning.
• We added an extra repr argument to control the repr. Let's emit a DeprecationWarning if it is passed.
• The default repr is different: <name> for us, just name for the builtin. Let's just change this.
• We made our Sentinel not picklable; the builtin is picklable by name. So let's add a __module__ attribute inferred from the caller (can reuse _set_module) and a __reduce__ that just returns self.__name__.
• The builtin doesn't support weakrefs; our version does because it's in Python. I don't know of a way to deprecate our support, so let's just document this difference.
• The builtin exposes a __name__ attribute; we have _name. Again let's just change this.
• There are some observable changes from the builtin being in C (e.g., the type is immutable), but let's not worry about that.

There are three open PRs about changing Sentinel. #715 and #732 make changes that introduce further incompatibilities, so I'm going to close those. #617 goes in the right direction but also introduces some further incompatible changes.

[JelleZijlstra]

Some more things:

• builtins.sentinel will raise TypeError on a non-string name; we accept it. Let's also raise DeprecationWarning for this.
• The name argument to builtins.sentinel will be positional only, but we currently accept it as a keyword arg. I think we can also DeprecationWarning this, though we'll have to complicate the signature a bit.
• builtins.sentinel has __copy__ returning itself, we don't.
• The current implementation also allows setting arbitrary instance attributes. See if we can DeprecationWarning this (in __setattr__ maybe?).