Python 3 wheels may depend on enum34

[pelme]

Using recent versions of setuptools, wheels will automatically be built locally and cached to improve installation speed.

However, if the wheel happens to be built with Python 2.7, it will have the enum34 dependency added to the wheel meta data.

Trying to install python-u2flib-server in another python 3.6 virtualenv will then install the python 2.7-built wheel, which will also pull in enum34.

The way to fix this issue is described here:
https://hynek.me/articles/conditional-python-dependencies/