Remove enum34 dependency

Author: Onur Rauf Bingol

requirements.txt

@@ -8,7 +8,6 @@ setuptools>=40.6.3
 six>=1.9.0
 
 # Modules ported to older Python versions
-enum34>=1.1.6; python_version < '3.4'
 backports.functools_lru_cache>=1.5; python_version < '3.3'
 typing; python_version < '3.5'