travis: run tests on macOS

.travis.yml

@@ -44,6 +44,21 @@ jobs:
       python: '3.7'
       sudo: required
       dist: xenial
+    - &test-macos
+      language: generic
+      os: osx
+      osx_image: xcode9.4
+      sudo: required
+      install:
+        - python -m pip install --pre tox
+      env: TOXENV=py27
+    - <<: *test-macos
+      env: TOXENV=py37
+      before_install:
+        - brew update
+        - brew upgrade python
+        - brew unlink python
+        - brew link python
 
     - stage: deploy
       python: '3.6'

changelog/3888.bugfix.rst

@@ -0,0 +1 @@
+Fix macOS specific code using ``capturemanager`` plugin in doctests.

src/_pytest/doctest.py

@@ -203,7 +203,8 @@ def _disable_output_capturing_for_darwin(self):
             return
         capman = self.config.pluginmanager.getplugin("capturemanager")
         if capman:
-            out, err = capman.suspend_global_capture(in_=True)
+            capman.suspend_global_capture(in_=True)
+            out, err = capman.read_global_capture()
             sys.stdout.write(out)
             sys.stderr.write(err)
 

testing/python/fixture.py

@@ -1584,6 +1584,7 @@ def test_package_fixture_complex(self, testdir):
             values = []
         """
         )
+        testdir.syspathinsert(testdir.tmpdir.dirname)
         package = testdir.mkdir("package")
         package.join("__init__.py").write("")
         package.join("conftest.py").write(