tox.ini: use system version of python3 for testing
    
    Since we need to use site-packages, we can't test py3X on any arbitrary
    system, just on the system where this exact version is the default one
    (so that all libraries are compiled with it). Py36 was the default on
    Fedora 28, but on Fedora 29, it's py37. To resolve this problem, specify
    just 'py3' as tox environment, which uses the default system python3
    version (so py36 on Fedora 28 and py37 on Fedora 29).
    
    Kudos to @mhroncok for the advice.