Move numpy out of requirements into extras, as it's not required
for running browbeat, but only used for insights like compare
results.
So to install with insights, pip install .[insights], to perform
the CLI operations such as compare.
Added a tox test to ensure no dep conflict arises.
Change-Id: Id8aafcd479003ae79ab8c2d0f1fa378ea38d60d2
Closes-Bug: #1799690