* Adds a minimal manual test that starts cqlsh with python3.7 & 3.8 in a Docker container. It also fixes a minor issue when starting up cqlsh with python3.7+, where `webbrowser._tryorder` is `None` as can be seen in https://github.com/python/cpython/blob/3.7/Lib/webbrowser.py#L1. Note that we're checking `webbrowser._tryorder` for `None` instead of doing `webbrowser.get()` because it's likely that there's no default browser installed in a Dockerized environment and so things would fail with `could not locate runnable browser` (https://github.com/python/cpython/blob/3.7/Lib/webbrowser.py#L65)
* Adds python3.8 testing to CircleCI
* Fixes some Python warnings that occur with newer versions
patch by Eduard Tudenhöfner, reviewed by Mick Semb Wever for CASSANDRA-15659
Andrés de la Peña