docs: prefer python_version attribute for specifying python version

[JeroenSchmidt]

This PR is updating the Examples and Docs relating to the recent changes refactor: fold per-target python version into base rules introduced in v.1.1.0.

See the brief conversation between myself and @rickeylev in this diff 611eda8

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[rickeylev]

Thanks! This looks pretty good overall. I'll send a few comments shortly.

Before we can accept the PR, though, you'll need to sign the CLA. https://cla.developers.google.com has instructions as to how. If you have already and the tool is mis-reporting, let me know, and I can force it to rescan.

[rickeylev]

A few typo corrections and minor word smithing, otherwise LGTM.

Just waiting for the CLA.