fix tests and move to sha512

Hello,

Tests were broken under the last version of both python (3.8) and CubicWeb.

This also remove the usage of md5 because in 2020 it is as secure as using cleartext of no password here.

This will likely breaks all tool that rely on Content-MD5 like cwclient lib.

I've also included some generic project practice modernization (black and check-manifest) along the way.

I've randomly assigned someone, please change the assignment if needed.

Kind regards,