Enforce use of python3 when doing computations

bin/czkrates

         test "$wntcur" = "$Code" || continue
         case $Amount in
             1) echo "${!wntfld}" ;;
-            *) python -c "print(float($Rate)/$Amount)" ;;
+            *) python3 -c "print(float($Rate)/$Amount)" ;;
         esac
     done
 }
         echo "$price"
     else
         think "multiplying by: $Mult"
-        python -c "print(float($price) * $Mult)"
+        python3 -c "print(float($price) * $Mult)"
     fi
 }