Do not require enum34 with Python 3.4 and above

enum34 is a backport of the enum library of Python 3.4 to earlier Python
versions, so enum34 is not required for Python 3.4 and above and should
not be installed.
diff --git a/requirements.txt b/requirements.txt
index 5f6aed1..5a81c5e 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,5 +1,5 @@
 pyparsing
 pyyaml
 six
-enum34
+enum34 ; python_version<'3.4'
 ddt