Support pip 20

[laike9m]

The lastest version of Dephell (0.8.3) requires pip<=19.3.1,>=18.0, and is not compatible with pip 20, whhich has been released for a while with many improvements.

[xbonnin]

+1

[NeilGirdhar]

When users upgrade to pip 20.3, dephell won't even be installable since the new pip rejects incompatible installs: https://pyfound.blogspot.com/2020/03/new-pip-resolver-to-roll-out-this-year.html.

[Eisbrenner]

I wasn't able to install dephell, I think partly due to this (and maybe some other issues with pyenv and macOS BS). Well, I ended up pipx install dephell which worked fine for now, since I'm not using dephell all that much. I assume that won't haunt me too soon... hopefully.