fix nested additionalProperties (spec-first#1138)

Merge pull request spec-first#1109 from spacether/issue_1108_py27_fix

Don't allow connexion installation in py27

use non-signed tag

drop Python 3.5, add 3.8 (spec-first#1056)

* drop Python 3.5, add 3.8

* remove old pypy 2.7 (no longer available on 18.04)