patch 7.4.1315

Problem:    Using a channel handle does not allow for freeing it when unused.
Solution:   Add the Channel variable type.

src/if_python3.c

@@ -1654,6 +1654,7 @@ do_py3eval (char_u *str, typval_T *rettv)
 	case VAR_FLOAT:
 	case VAR_SPECIAL:
 	case VAR_JOB:
+	case VAR_CHANNEL:
 	    break;
     }
 }