"Backport" FileNotFoundError to Python 2

Author: rayluo

tests/test_client.py

@@ -6,6 +6,10 @@
 except:  # Python 3
     from urllib.parse import urljoin
 import time
+try:
+    FileNotFoundError  # Python 3
+except NameError:
+    FileNotFoundError = IOError  # Python 2
 
 import requests