ci: Travis: test with pexpect on macos

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

Author: blueyed

.travis.yml

@@ -33,7 +33,7 @@ jobs:
         - python -V
         - test $(python -c 'import sys; print("%d%d" % sys.version_info[0:2])') = 27
     - <<: *test-macos
-      env: TOXENV=py37-xdist
+      env: TOXENV=py37-pexpect-xdist PYTEST_COVERAGE=1
       before_install:
         - which python3
         - python3 -V