QoL: Fix tox.ini syntax and other minor things

QoL: Fix tox.ini syntax and other minor things

- Fix tox.ini syntax

Modern tox versions require the expected environment variables to be
listed one by one on separate lines in tox.ini

- Add `venv` to list of ignored names for git

This is a common name for a local Python virtual environment. Less
typing than `virtualenv`

- Add `tox` to requirements.txt

Installing `tox` via `pip` has the advantage of including it in the
virtual environment, thus avoiding trouble on operating systems shipping
by default with python3.6 or older. It's also nice that `pip install -r
requirements.txt` is now sufficient to set up the testing environment,
after initializing the virtual environment with a moder-enough python
version.

Signed-off-by: Moritz Röhrich <moritz.rohrich@suse.com>