`Sequence` and `Mapping` are commonly used in argument types. This can
be problematic, since they are not protocols, making it impossible to
safely duck type or mock them. Also, not all sequence- or mapping-like
types derive from these ABCs at runtime, although in typeshed we often
pretend they do. Ideally we'd be able to avoid this discrepancy in the
far future. Finally, the ABCs sometimes make more API promises than
their sub-classes fulfill. For example, `Mapping` arguments are
keyword-or-positional, while its most important subtype `dict` only
accepts positional arguments. These protocols have tighter guarantees.

`SequenceLike` contains most, `MappingLike` all methods from their
respective types, making them an easy substitute in argument types.