mypy plugin: More precisely detect when fields are required.

[richardxia]

Change Summary

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.

Related issue number

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100% (Coverage was not at 100% to begin with for me, but the coverage did not drop after I made my changes)
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[richardxia]

Whoops, I misunderstood the last item on the checklist. Please review when you have the time.

[samuelcolvin]

great, thanks so much.

[richardxia]

Thanks!