Update Python 2 examples in docs to Python 3

[dosisod]

I recently came across this library and saw that it still had Python 2 references in the docs. I saw #523 was open, so I went ahead and updated the documentation to use Python 3 examples.

Once #523 is merged this could be merged as well.

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(Out of scope, but leaving this here for now, could make this a separate issue)

Now that we're dropping Python <=3.7 there are a lot of Python 3 related changes and quality of life improvements we can now make:

• Add type annotations
• Remove remaining Python 2 compatibility checks
• Improve testing infrastructure

I wouldn't mind working on these in my free time if we decide these would be useful!

[JonathanHuot]

Will be happy to merge any code drop of py2; thanks for this one already !