Uninstall enum34

This is a holdover from long since passed Python 2.7 days. It probably should have been removed long ago. `enum34` is a backport of the stdlib enum module for Python versions < 3.4, so we no longer need it since we require 3.6.

Error seen attempting to `pip install psycogreen==1.0.2` when enum34 is installed:

AttributeError: module 'enum' has no attribute 'IntFlag'

Relevant links:
https://stackoverflow.com/q/43124775/10840
pypa/pip#8214

requirements/uninstall-requirements.txt

@@ -15,3 +15,4 @@ subprocess32
 eventlet
 contextlib2
 redis-py-cluster
+enum34