Update events handling documentation to work with Python 3.

[pganssle]

Event handling documentation has a lot of Python 2-style print statements, so I have updated them here. I ran the code snippets in Python 3 and they all work. The only difference is that for the Python 3 version, the outputs look like this:

'event contains', (7.0, 0.0)

And for Python 2 they look like this now:

('event contains', (7.0, 0.0))

They used to not have parens around the outputs, but they do now because of the parens required to make it polyglot code. The old behavior can return if you prepend from __future__ import print_function, but I don't really think that's necessary.

[tacaswell]

👍 It looks like the docs failure is due to poor version checking in the link checker.

[pganssle]

Is the coverage failure messed up? It's talking about changes in coverage that didn't have anything to do with what I changed.

[tacaswell]

backported to v1.5.1-doc as 520d1a1

[tacaswell]

I closed and re-opened to re-trigger the builds. I do not know what it's base line is, but it might be looking at the first time it ran this PR vs the second time it ran this PR against a different master.