doc: remove references to python 2

doc: remove references to python 2

Python 2 support has now been dropped. Remove references to it in the
documentation.

Since all python scripts now have a proper shebang that calls python3,
execute the scripts directly without specifying the interpreter.

Sphinx version from most Linux distros is OK in 2020, do not encourage
people to break their system by installing with pip. Use the distros
official packages.

Signed-off-by: Robin Jarry <robin.jarry@6wind.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Kevin Laatz <kevin.laatz@intel.com>