fix: use env marker to install enum34

Rely on [environment markers](https://www.python.org/dev/peps/pep-0508/#environment-markers)
to conditionally install enum34.

Author: hadrien

setup.py

@@ -28,6 +28,7 @@ def find_version(*file_paths):
     long_description = f.read()
 
 install_requires = [
+    'enum34  ; python_version < "3.4"',
     'requests',
     'six',
     'pytz',