[examples] add python3 catch for examples doc script

This adds an additional catch to use the script for the documentation of examples with python3.

I am not entirely sure if we really need this as the shebang is already there. So if users/developers use the script correctly, the catch is not needed. However, if as in #952 (closed) the script is used with python directly, the python generated error-message is not really helpful.

Let's discuss this, before we merge!

So this MR is here in case we think this is catch is of any help.

fixes #952 (closed)