Travis: use osx-py27-xdist for coverage instead of py37-xdist

Author: blueyed

.travis.yml

@@ -21,12 +21,14 @@ jobs:
       os: osx
       osx_image: xcode10.1
       language: generic
-      env: TOXENV=py27-xdist
+      # Coverage for:
+      # - py2 with symlink in test_cmdline_python_package_symlink.
+      env: TOXENV=py27-xdist PYTEST_COVERAGE=1
       before_install:
         - python -V
         - test $(python -c 'import sys; print("%d%d" % sys.version_info[0:2])') = 27
     - <<: *test-macos
-      env: TOXENV=py37-xdist PYTEST_COVERAGE=1
+      env: TOXENV=py37-xdist
       before_install:
         - which python3
         - python3 -V