Deep dict type-checking x 4.

This commit is the epochal next commit in a commit chain deeply
type-checking the first key-value pair of each **mapping type hint**
(e.g., of the form `dict[..., ...]`, `collections.defaultdict[...,
...]`, `collections.abc.Mapping[..., ...]`,
`collections.abc.MutableMapping[..., ...]`,
`collections.abc.OrderedDict[..., ...]`, `typing.Dict[..., ...]`,
`typing.Mapping[..., ...]`, `typing.MutableMapping[..., ...]`, or
`typing.OrderedDict[..., ...]`) in `O(1)` time, en-route to partially
resolving long-standing feature requests #167 kindly submitted sometime
during my most recent past life by ardent typing fiend @langfield *and*
#2021 kindly submitted sometime during the past life immediately
preceding my most recent past life by Equinor ASA bear bro @jondequinor
(Jonas Grønås). Specifically, this commit exhaustively exercises this
type-checking with PEP 484-compliant type hints and sample data. This
*almost* concludes this commit chain. Now, we only have to exercise this
type-checking with PEP 585-compliant type hints and... *we will be
done.* (*Chilly chalice ain't painfully frilly!*)