imports: Fix urllib3 exception import

Make it compatible with vendoring: import the exception only to avoid
having to rename the module locally.

Signed-off-by: Jussi Kukkonen <[email protected]>

Author: Jussi Kukkonen

tuf/requests_fetcher.py

@@ -10,8 +10,7 @@
 import six
 import logging
 import time
-
-import urllib3.exceptions
+from urllib3.exceptions import ReadTimeoutError
 
 from tuf import exceptions
 from tuf import settings
@@ -120,7 +119,7 @@ def chunks():
           if bytes_received >= required_length:
             break
 
-      except urllib3.exceptions.ReadTimeoutError as e:
+      except ReadTimeoutError as e:
         raise exceptions.SlowRetrievalError(str(e))
 
       finally: