CircleCI: Python 3 container: Create symlinks for pylint & pytest
CircleCI: Python 3 container: Create symlinks for pylint & pytest

This is easier than changing all our self-test code to select between
`pylint3` for Python 3 or `pylint` for Python 2.

With this, `make check-lint` is now running and reporting all sorts of
problems. Progress!

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester