gallery.py: Add a 'version' argument

gallery.py: Add a 'version' argument

With the 'version' action [1], instead of passing the version as an
argument to ArgumentParser (which was at one point supported in
Python, but I can't find the deprecation docs).

[1]: https://docs.python.org/3/library/argparse.html#action
[2]: https://docs.python.org/3/library/argparse.html#argparse.ArgumentParser