Fix macOS specific code that uses capturemanager.

https://github.com/pytest-dev/pytest/issues/3888#issuecomment-416206606

closes #3888

Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>

changelog/3888.bugfix.rst

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

src/_pytest/doctest.py

@@ -203,7 +203,8 @@ class DoctestItem(pytest.Item):
             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)