fix: import Mapping from typing module

[roedoejet]

Panphon > 0.20.0 is currently failing on python 3.8

in python 3.8 you cannot subscript the collections.abc.Mapping class so importing from typing is more stable across python versions.

see https://stackoverflow.com/questions/59955751/abcmeta-object-is-not-subscriptable-when-trying-to-annotate-a-hash-variable