Detox

- as tox added a parallel mode in 3.7 detox will only work with older versions of tox now so the installation requires a version of tox < 3.7
- mark repository as unmaintained
- add a warning at the end of the testrun hinting the user to try it without detox and a version of tox > 3.6

Note that usedevelop still does not seem to be fixed (PRs welcome)

- integrate usable fixes from stale PR7
- (fix [20](https://github.com/tox-dev/detox/issues/20) detox
ignores/breaks usedevelop - by Kendall Chuang
- (fix [21](https://github.com/tox-dev/detox/issues/21)
detox doesn't support skipsdist config option - by Kendall Chuang
- convert changelog to markdown and render as part of description on PyPI
- detox version now prints own version and then what tox has to say
(it's a plugin after all and that should be made clear)
- [Internal changes]
- add extra dependencies in setup.py
- update tests to current pytest API
- use black for code formatting
- use flake8 for linting
- add descriptions to tox envs
- add a "dev" tox env

- (fix [23](https://github.com/tox-dev/detox/issues/23)) do not
swallow exceptions - by vlaci
- (fix [16](https://github.com/tox-dev/detox/issues/16)) use tox
quiet level to make detox quiet - by Oliver Bestwalter

- and now the markdown description might even be rendered