Do not raise UsageError when "pytest_plugins" is a module

Fix #3899

changelog/3899.bugfix.rst

@@ -0,0 +1 @@
+Do not raise ``UsageError`` when an imported package has a ``pytest_plugins.py`` child module.

src/_pytest/config/__init__.py

@@ -559,8 +559,8 @@ def _get_plugin_specs_as_list(specs):
     which case it is returned as a list. Specs can also be `None` in which case an
     empty list is returned.
     """
-    if specs is not None:
-        if isinstance(specs, str):
+    if specs is not None and not isinstance(specs, types.ModuleType):
+        if isinstance(specs, six.string_types):
             specs = specs.split(",") if specs else []
         if not isinstance(specs, (list, tuple)):
             raise UsageError(

testing/acceptance_test.py

@@ -969,6 +969,20 @@ def test_import_plugin_unicode_name(testdir):
     assert r.ret == 0
 
 
+def test_pytest_plugins_as_module(testdir):
+    """Do not raise an error if pytest_plugins attribute is a module (#3899)"""
+    testdir.makepyfile(
+        **{
+            "__init__.py": "",
+            "pytest_plugins.py": "",
+            "conftest.py": "from . import pytest_plugins",
+            "test_foo.py": "def test(): pass",
+        }
+    )
+    result = testdir.runpytest()
+    result.stdout.fnmatch_lines("* 1 passed in *")
+
+
 def test_deferred_hook_checking(testdir):
     """
     Check hooks as late as possible (#1821).