old_contents: from .base import Invalid, Missing, Validator, Anything
  from .scalar import Scalar, ObjectId, Number, Integer, Float, Unicode
  from .compound import Document, Array
  from .compiler import compile_schema
new_contents: from .base import Invalid, Missing, Validator, Anything
  from .scalar import Scalar, ObjectId, Number, Integer, Float, Unicode, DateTime
  from .compound import Document, Array
  from .compiler import compile_schema
  
subject: Add missing import