light-the-torch version 0.4.0 incompatibility

[danodonovan]

The latest light-the-torch package has refactored some internal APIs and tox-ltt now fails with the following error

 File ".tox/.tox/lib/python3.7/site-packages/tox_ltt/plugin.py", line 11, in <module>
    from light_the_torch.cli import make_ltt_parser
ModuleNotFoundError: No module named 'light_the_torch.cli'

Unless we pin light-the-torch (in tox.ini)

requires =
    tox-ltt
    light-the-torch<0.4.0

I hope to submit a patch for this soon :-)