Some cleanups for Python code

Some cleanups for Python code

Individual changes:
1) Update documentation for Python 3

2) Use gettext.install() for translations

This conveniently installs _() into Python's builtins namespace.

3) Drop require_python_version()

The implemented 'help' for the user by opening the browser does
not work on Linux anyway.

4) Suggested changes for a newer version of Python

Some comments already referred to simpler code, let's do it.

[lemzwerg, 2020-02-02 13:48:27]

LGTM