use builtin_type if it exists, otherwise named_type

[shawnwall]

resolves https://github.com/dropbox/sqlalchemy-stubs/issues/232

• mypy .930 deprecated builtin_type in favor of named_type, but brought it back in .931
• this patch uses hasattr() to check for existence of builtin_type, and if not present, use named_type for future proofing code