Fix for docplex 2.20

Author: t-imamichi

qiskit/optimization/problems/quadratic_program.py

@@ -24,7 +24,7 @@
 from docplex.mp.constr import (LinearConstraint as DocplexLinearConstraint,
                                QuadraticConstraint as DocplexQuadraticConstraint,
                                NotEqualConstraint)
-from docplex.mp.linear import Var
+from docplex.mp.dvar import Var
 from docplex.mp.model import Model
 from docplex.mp.model_reader import ModelReader
 from docplex.mp.quad import QuadExpr