Allow flakehell to be installed from the source alone

[thejcannon]

See https://flit.readthedocs.io/en/latest/pyproject_toml.html#build-system-section

This will allow flakehell installable from just the source. E.g. pip install git+git+https://github.com/life4/flakehell

[thejcannon]

I can't say I understand the ecosystem of dephell and flit enough to diagnose the CI failures.
The relevant info seems to be:

$ dephell deps install --env=$ENV
INFO get dependencies (format=flit, path=pyproject.toml)
INFO build dependencies graph... 
ERROR RecursionError: maximum recursion depth exceeded while getting the str of an object 

This seems to me like a dephell issue.

[mjpieters]

The build failures on Travis are not due to this pull request; a simple change to a config file also fails.

[mjpieters]

I can reproduce this locally, and its indeed a dephell issue, which hits an infinite recursion case when trying to test against isort. This wasn't an issue before, but isort released 5.0.0, 5.0.1, 5.0.2, 5.0.3 and 5.0.4 in the past few days. I'm seeing what needs to be done here to avoid this mess.

[mjpieters]

See #90; the trick is to pin isort.

[thejcannon]

@mjpieters Thanks for the diagnosis and solution! I'm subscribed to that PR and will pull the latest changes when it gets in 👍

[orsinium]

Great! Thank you all :)