Topic/explicit python env

@nnrcschmdt I've done some small improvements:

• using "virtualenv" instead of "venv" in the Readme, to be consistent with all the other Readmes
• using an explicit Python version (3.8) für vitualenv, because "python3" uses whatever is available
• Updated the "init" script in "run.sh"

If you are fine with them would be great if you could merge. Thanks :)

README.rst

@@ -37,7 +37,7 @@ Setting up the environment
 
 Create a virtual environment where the dependencies will live::
 
-    $ virtualenv -p python3.8 python
+    $ python3.8 -m venv python
     $ source python/bin/activate
     (python)$