Ensure field test targets inherit from Document.
Add flake8.
Fix flake8 errors for documents.py.
Clean up flake8 errors in errors.py.
Clean up flake8 errors in __init.py__.
Clean up flake8 errors in fields.py.
Clean up flake8 errors in utils folder.
Fix flake8 errors in base folder.
Clean up flake8 errors in test files.