Cast a long integer to double explicitly.

modules/python/src2/cv2.cpp

@@ -233,7 +233,12 @@ static int pyopencv_to(const PyObject* o, Mat& m, const ArgInfo info, bool allow
 
     if( PyInt_Check(o) )
     {
-        double v[] = {PyInt_AsLong((PyObject*)o), 0., 0., 0.};
+        double v[] = {
+            static_cast<double>(PyInt_AsLong((PyObject*)o)),
+            0.,
+            0.,
+            0.,
+        };
         m = Mat(4, 1, CV_64F, v).clone();
         return true;
     }