use NewType instead of str everywhere

https://docs.python.org/3/library/typing.html#newtype

@dosu what do you think.

Read the linked Python typing.NewType documentation first, then inspect the repository’s Python code to determine what “everywhere” covers. Define the affected annotations and compatibility boundaries before making changes; done should mean the agreed scope consistently uses NewType and the relevant checks still pass.

Written by the indexing model from the issue text.