configparser: allow ConfigParser()[UNNAMED_SECTION]

[trillllian]

This probably should have been part of #13544, but seems to have been forgotten.

Fixes a type error on the last line of:

import configparser
config = configparser.ConfigParser(allow_unnamed_section=True)
config.read_string("a = b")
config[configparser.UNNAMED_SECTION]

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[srittau]

Thanks!