Fix another legacy Python compatibility issue

thriftpy2/hook.py

@@ -3,18 +3,16 @@
 from __future__ import absolute_import
 
 import sys
-import importlib.util
-import types
 
 from .parser import load_module
 
 
-
 # TODO: The load process does not compatible with Python standard, e.g., if the
 # specified thrift file does not exists, it raises FileNotFoundError, and skiped
 # the other meta finders in the sys.meta_path.
 if sys.version_info >= (3, 4):
     import importlib.abc
+    import importlib.util
 
     class ThriftImporter(importlib.abc.MetaPathFinder):
         def __init__(self, extension="_thrift"):