Drop enum34 in favor of stdlib enum

Using enum34 has been unnecessary since dropping Python 2 support. It
previously remained as a compatibility shim.

Author: jdufresne

tools/requirements/tests.txt

@@ -1,6 +1,5 @@
 --use-feature=2020-resolver
 cryptography==2.8
-enum34; python_version < '3.4'
 freezegun
 mock
 pretend