typing.Mapping is not a protocol, which has caused problems in the past.
(E.g. python/typeshed#3569, see also python/typeshed#3576.) This
introduces a few narrow protocols to _typeshed.pyi that can be used in
place of Mapping.
Not all uses of Mapping can be replaced. For example, cgi.FieldStorage
explictly checks whether the supplied headers argument is a Mapping
instance.
