ci: Travis: test with pexpect on macos
ci: Travis: test with pexpect on macos

Uses `PYTEST_COVERAGE=1` already, which will come in via dropping
Python 2 soon anyway.

tomviner/pytest,markshao/pytest,RonnyPfannschmidt/pytest,nicoddemus/pytest,tomviner/pytest,pytest-dev/pytest,alfredodeza/pytest,nicoddemus/pytest,Akasurde/pytest,The-Compiler/pytest,The-Compiler/pytest