Calling Python from Lean

[katjabercic]

We should look for a reasonable way for the user to set the path to the Python executable.

As noted by @GauvainD:

Note about python in general: on my computer, the command is python3 and lean does not seem to consider aliases. It might be a plus to have some general config file where we can set which python command to use.

@cilinder solved this partly and temporarily by first checking for a python command and then python3. This does not solve the issue when the user wants to use the virtual environment's Python executable.