ci: Travis: test with pexpect on macos

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

.travis.yml

@@ -20,9 +20,6 @@ jobs:
   include:
     # OSX tests - first (in test stage), since they are the slower ones.
     - &test-macos
-      # NOTE: (tests with) pexpect appear to be buggy on Travis,
-      #       at least with coverage.
-      #       Log: https://travis-ci.org/pytest-dev/pytest/jobs/500358864
       os: osx
       osx_image: xcode10.1
       language: generic
@@ -33,7 +30,7 @@ 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,py37-xdist PYTEST_COVERAGE=1
       before_install:
         - which python3
         - python3 -V