Make `TagAttrs` a `Mapping`

[schloerke]

          I think a better way to go about this is to change `TagAttrs` from a `Dict` to a `Mapping`. (The reason it didn't accept a `TagAttrDict` as a `TagAttrs` object is because `Dict` is invariant, whereas `Mapping` is covariant.)

TagAttrs = Mapping[str, TagAttrValue]

There is also a place where you'll have to test for isinstance(x, Mapping) instead of isinstance(x, dict).

Originally posted by @wch in #55 (comment)