Bug in Expression.satisfy_one with assumptions

[TheKiviest]

I call satisfy_one with assumptions for expression which is not in CNF form. Then it calls _backtrack here, receives None because restricted expression is unsatisfiable and crashes at next line.

Something like this would fix the problem:

soln = _backtrack(self.restrict(aupnt))
return None if soln is None else soln.update(aupnt)