Fix validate python3 version of x-ray

[SandraPico]

Documentation refers to "pip install x-ray dependencies" which can install the x-ray dependencies for python 2 version. This breaks api. Change documentation to pip3 install and see if we can verify the python x-ray package version to be 3 or raise an error otherwise.

[kalleeh]

Docs are updated to reflect pip3 command but no validation is in place.