The mypy plugin would previously incorrectly determine that a field was
not required in a few scenarios where the field really is required. This
specifically affected cases when the `Field()` function is used, where
the plugin assumed that the first argument would always be `default`.
This changes the code to examine each argument more closely, and it now
properly handles several more scenarios where the default is explicitly
named or when the default_factory named argument is used.
Richard Xia