test/py: Update docs, add requirements.txt for pip

test/py: Update docs, add requirements.txt for pip

To be more closely aligned with Python community best practices, we need
to better document our usage of pip and make use of a requirements.txt
file that shows the versions of the tools that we are using.  This will
aide in ensuring reproducibility of our tests as well.

Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Simon Glass <sjg@chromium.org> [on sandbox]
Signed-off-by: Tom Rini <trini@konsulko.com>