Avoid warning on complex-to-float conversion (quantumlib#5795)

Make it clear we want to use the real part.

Author: pavoljuhas

cirq/ops/pauli_string.py

@@ -745,7 +745,7 @@ def _expectation_from_density_matrix_no_validation(
         while any(result.shape):
             result = np.trace(result, axis1=0, axis2=len(result.shape) // 2)
 
-        return float(result * self.coefficient)
+        return float(np.real(result * self.coefficient))
 
     def zip_items(
         self, other: 'cirq.PauliString[TKey]'