def guess_type_from_validators(attr):
return _guess_type_from_validator(attr.validator)<EOL>
Utility method to return the declared type of an attribute or None. It handles _OptionalValidator and _AndValidator
in order to unpack the validators.

:param attr:
:return: the type of attribute declared in an inner 'instance_of' validator (if any is found, the first one is used)
or None if no inner 'instance_of' validator is found