Installation depends on testfixtures

[sebbacon]

setup.py only requires it for tests, but actually it's required for normal operations, resulting in:

$ jupytext mynoteboook.ipynb  --to md --output -
Traceback (most recent call last):
  File "/home/seb/.local/share/virtualenvs/jupyter/bin/jupytext", line 7, in <module>
    from jupytext.cli import jupytext
  File "/home/seb/.local/share/virtualenvs/jupyter/local/lib/python3.6/site-packages/jupytext/cli.py", line 9, in <module>
    from .compare import test_round_trip_conversion
  File "/home/seb/.local/share/virtualenvs/jupyter/local/lib/python3.6/site-packages/jupytext/compare.py", line 3, in <module>
    from testfixtures import compare
ModuleNotFoundError: No module named 'testfixtures'

[mwouts]

Thanks @sebbacon for reporting. I'd be happy to fix this in the next release. Would you like to submit your change as a pull request? Thanks