[configparser] Narrow RawConfigParser.values() to SectionProxy - #16238

Closes: #11547

RawConfigParser already narrows __getitem__ and items() to SectionProxy but never declared
values(), so it falls through to Mapping.values with _VT_co bound to _Section.

Deriving the class from MutableMapping[str, SectionProxy] instead would regress code that checks
clean today:

error: Dict entry 0 has incompatible type "str": "dict[str, str]"; expected "str": "SectionProxy"  [dict-item]
error: Argument 2 to "setdefault" of "MutableMapping" has incompatible type "dict[str, str]"; expected "SectionProxy"  [arg-type]

on cp.update({"a": {"x": "1"}}) and cp.setdefault("b", {"y": "2"}), both of which work at
runtime. _Parser is also the declared parameter type of the four Interpolation hooks, so
narrowing it breaks third-party subclasses.

popitem() has the same gap and is narrowable on its own, so I left it for a separate PR.
setdefault() isn't: it returns the caller's own object, not the proxy it stored.

Assisted by Claude Code.