Merge pull request #5075 from certbot/specify-min-six-version

Specify the minimum six version in ACME

acme/setup.py

@@ -20,7 +20,7 @@ install_requires = [
     # For pkg_resources. >=1.0 so pip resolves it to a version cryptography
     # will tolerate; see #2599:
     'setuptools>=1.0',
-    'six',
+    'six>=1.9.0',  # needed for python_2_unicode_compatible
 ]
 
 # env markers cause problems with older pip and setuptools