[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)