kudu-python: clarify and document on supported Python versions

Due to the widely circulating information on the planned obsolescence of Python2, there have been inquiries about versions of Python supported by kudu-python package.

As it seems, running applications based on kudu-python using Python3 has been supported for a very long time. Anyway, it would be nice to clarify and confirm on the requirements w.r.t. using Python2/Python3 for building and running kudu-python.

It's necessary to document the findings on the supported Python versions in the user-facing documentation such as documentation on Kudu Python client at the Kudu website.