SimpleFormValidator doesn't work with sc hema.from_python({}, None)

 Hi,

 Should SimpleFormValidator inherit from FormValidator instead of 
 FancyValidator? Otherwise, when used as a chained validator, a call to 
 schema.from_python({}, None) will raise an exception, as 
 FancyValidator.from_python({}, None) returns None and fails the dict 
 assertion. OTOH, FormValidator.from_python({}, None) returns {} as is.

 Regards,
 Yap

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