Explicate dependency on python3-dev

somewhat a reminder for myself to start a pull request.

When installing, I noticed that not only python3 is needed, but also python3-dev installed via sudo apt install python3-dev, to get the Python headers (to be able to install netifaces).

I used debian with python3.8.

When doing this, I had to lift the version requirement of web.py in the file "requirements.txt", too. Otherwise, the app would not start up with error:

RuntimeError: generator raised StopIteration. (this was a breaking change in behavior, introduced in Python 3.7)