Use vendored urllib3.

As long as requests ships with urllib3, we can't fall back to the
"system" urllib3, since that breaks class-identity checks.

dosagelib/util.py

@@ -18,10 +18,7 @@ except ImportError:
     import robotparser
 import requests
 from requests.adapters import HTTPAdapter
-try:
-    from urllib3.util.retry import Retry
-except ImportError:
-    from requests.packages.urllib3.util.retry import Retry
+from requests.packages.urllib3.util.retry import Retry
 import sys
 import os
 import cgi