move python examples to python3

Right now, some examples are written in Python 2, some in 3. Most scripts are missing a shebang.

I'll update the examples to Python 3 syntax and perform some cleanups unless someone stops me :-)

Inspect the repository's example scripts to identify which use Python 2 syntax and which lack shebangs. Run the examples with Python 3 where possible, and consider the work done when the examples consistently use Python 3 syntax and include shebangs.

Written by the indexing model from the issue text.