This is a second try for #251.

This is not changing the isfile, so the issue with No such file or directory: 'hilo_state.yaml' should not happen again.

I also found an issue when doing from __future__ import annotations that would break the dict detection in _get_defaults. I left a bunch of comments explaining what the code is doing, since it's a bit obscure.

D100 Missing docstring in public module
UP035 typing.Type is deprecated, use type instead
D101 Missing docstring in public class
FA100 Add from __future__ import annotations to simplify typing.Optional
N807 Function name should not start and end with __
UP006 Use type instead of Type for type annotation
D400 First line should end with a period
D401 First line of docstring should be in imperative mood: "Generates a default dict based on typed dict"
D415 First line should end with a period, question mark, or exclamation point
PGH003 Use specific rule codes when ignoring type issues
D205 1 blank line required between summary line and description
FA100 Add from __future__ import annotations to simplify typing.Union

Not done in this PR:
PTH113 os.path.isfile() should be replaced by Path.is_file()

This is not really needed, I feel it would make the code harder to read and it would make performance a tiny fraction worse too, so let's not do that since we don't need anything else from the Path object.