Remove deprecated _field_types attribute for Python 3.9

[mathom]

The _field_types attribute was marked as deprecated for a while and was removed in favor of using __annotations__ in Python 3.9: https://bugs.python.org/issue40182

I also had to fix some of the Mypy test formatting because the error messages were changed to use double quotes python/mypy#7445

[NazarioJL]

@mathom see #33 improves a bit on your PR. I need to see with the maintainers if they are ok with formatting the whole project with black, otherwise I'll just remove the black hook from pre-commit.

[fsouza]

Fixed by #33 😁