Clean xsdbase module after testing  __slots__
Clean xsdbase module after testing  __slots__

  - Add __slots__ to XSD validator classes seems to be problematic.
    Having default values for these class hierarchy is important to
    simplify the inner code. To keep flexibility also '__dict__'
    should be added to __slots__, so the memory saving would be
    pretty small.

sissaschool/xmlschema