black and isort fit into the category of linters.
Let's run everything from the same job so we can avoid wasting time
and resources from the CI infrastructure.
Previously, tox would exit immediately if there was an error with
black, preventing isort from running. You could then run into a
situation where you also had an issue picked up by isort.
Now bash will run all linters and return the appropriate
return code at the end.
Change-Id: I81abe61be65bd9f4f828cfc5f0b7a1d9f1eb1f22
1f205e2ad9