Make tox work (just runs pytest for now)
    
    This involves twiddling quite a few other moving parts to make
    everything work without unnecessary duplication. I didn't go
    all the way to PEP 517 compatibility yet. Tested that `tox`,
    `make test` and `make test-ci` all do something useful. Also
    added some missing bits to .gitignore.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>