gnu: Don't use python-enum34 when building with python-3.4.

* gnu/packages/docker.scm (docker-compose)[inputs]: Remove python-enum34.
* gnu/packages/python.scm (python-parse-type)[inputs]: Likewise.
(behave)[inputs]: Likewise.
(python-cryptography)[propagated-inputs]: Likewise.
(python2-cryptography)[propagated-inputs]: Add python2-enum34.