circleci: Ubuntu 18.04: Install `responses` and `pytest` from apt
circleci: Ubuntu 18.04: Install `responses` and `pytest` from apt

Ubuntu 18.04 has python-responses==0.5.1 and pytest==3.3.2.

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