Drop indirect dependencies.

### Motivation
We depend on packages who depend on other packages. However, it is not our task to manage the versions of the indirect dependencies; the packages should manage these themselves and install whatever version they see fit, or reuse an existing version found in the user's system.

### Changelog
* Drop `backports.ssl-match-hostname`, `certifi`, `ecdsa`, `websocket.client`, `wsgiref`, `zope.interface`.
* Install a temporary `pip freeze` at the end of Travis build to showcase that some of the indirect dependencies are installed.