Fix failing py3 builds attempt 4

Author: pevdh

telebot/apihelper.py

@@ -5,7 +5,7 @@
 import types
 
 from telebot import util
-from util import logger
+from telebot import logger
 
 API_URL = "https://api.telegram.org/bot{0}/{1}"
 FILE_URL = "https://api.telegram.org/file/bot{0}/{1}"

telebot/util.py

@@ -4,7 +4,6 @@
 import re
 import sys
 import six
-from six import string_types
 
 import logging
 
@@ -160,7 +159,7 @@ def wrapper(*args, **kwargs):
 
 
 def is_string(var):
-    return isinstance(var, string_types)
+    return isinstance(var, six.string_types)
 
 
 def is_command(text):