Allow installation in virtualenv.

Today, when checking the latest master for Qutepart, I noticed that actually it's not possible to install Enki in a Virtualenv. This fixes it, with Python 3 forward compatibility.

Now, to just test the latest Enki and Qutepart, one can just do:

    # First install the dependencies from your system; then...
    virtualenv --system-site-packages env-dir
    source env-dir/bin/activate
    pip install https://github.com/hlamer/enki/archive/master.zip https://github.com/hlamer/qutepart/archive/master.zip
    ./env-dir/bin/enki

Easier to test!