Update requirements.txt (#660)

Summary:
enum34; python_version

Pull Request resolved: #660

Reviewed By: mengxuanzhangz

Differential Revision: D53016840

Pulled By: stcheng

fbshipit-source-id: f454c643361235b73a6ced05b3a9ab3c3744f408

requirements.txt

@@ -2,5 +2,5 @@ requests >= 2.3.0
 six >= 1.7.3
 curlify >= 2.1.0
 pycountry >= 19.8.18
-enum34; python_version < '3.4'
+enum34; python_version >= '3'
 aiohttp; python_version >= '3.5.3'