[DOCS] Fix trivup example, now requires version flag, update Kafka example test version to current

[mdb-ebeebe]

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It looks like the syntax for the trivup module has changed since the README was written, and it now requires a '--version' flag to specify the version.

Also, the example version was very old, so I updated the (example) to a currently supported for (3.9.0). In testing, the tests that pass correctly under 2.3.0 for me also pass under 3.9.0, so I wouldn't expect this to cause any problems, but users are still fine to specify the old version(s) if necessary.

I also added a warning about Python versions (eg. when trying this example with 3.6.9, it fails with the example error provided - it looks like from the release notes, the importlib feature which trivup relies on was introduced in Python 3.7, and I verified by testing several Python versions).

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