[configparser] Narrow RawConfigParser.popitem() to SectionProxy - #16289

Follow-up to #16238, which left this one out.

RawConfigParser never declares popitem(), so it resolves to MutableMapping.popitem with _VT bound to _Section:

cp = configparser.ConfigParser()
cp.read_string("[a]\nx = 1\n")
name, section = cp.popitem()
section.name

error: "Mapping[str, str]" has no attribute "name"  [attr-defined]

At runtime RawConfigParser defines its own popitem, and it returns self[key], which __getitem__ already declares as SectionProxy. __getitem__, items() and values() are all narrowed the same way.