Release v1.0.3

Updates changelog and marks new release version.

Release v1.0.2 for packaging

Updates some docs and bumps the version number.

Fix mypy warning

Our new type formatting lookup table dict is using `.get(val, str)`
where `str` is the fallback class we'd use if no direct matches are
found, but mypy finds it confusing returning a type constructor in place
of everything else.

Manual type override helps mypy be less confused about our usecase here.

Update changelog for version 1.0.0

Also doing more README maintenance.