Uninstall enum34

[millerdev]

Fixes a bug in pip install during deploy. Found while attempting to deploy #28487 (and other changes).

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, which applies to some of machines on prod but not all of them:

Traceback (most recent call last):
  File "/usr/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "lib/python3.6/site-packages/pip/__main__.py", line 23, in <module>
    from pip._internal.cli.main import main as _main  # isort:skip # noqa
  File "lib/python3.6/site-packages/pip/_internal/cli/main.py", line 5, in <module>
    import locale
  File "lib/python3.6/locale.py", line 16, in <module>
    import re
  File "lib/python3.6/re.py", line 142, in <module>
    class RegexFlag(enum.IntFlag):
AttributeError: module 'enum' has no attribute 'IntFlag'

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

[millerdev]

I think it should be safe to merge before tests have run since enum34 is not a requirement, and therefore the change in this PR will have no affect on tests.