Merge remote-tracking branch 'upstream/master' into mm

Conflicts:
	src/_pytest/main.py
	src/_pytest/mark/structures.py
	src/_pytest/python.py
	testing/test_main.py
	testing/test_parseopt.py

src/_pytest/config/__init__.py

@@ -28,7 +28,6 @@ from pluggy import HookspecMarker
 from pluggy import PluginManager
 
 import _pytest._code
-import _pytest.assertion
 import _pytest.deprecated
 import _pytest.hookspec  # the extension point definitions
 from .exceptions import PrintHelp
@@ -284,6 +283,8 @@ class PytestPluginManager(PluginManager):
     """
 
     def __init__(self):
+        import _pytest.assertion
+
         super().__init__("pytest")
         # The objects are module objects, only used generically.
         self._conftest_plugins = set()  # type: Set[object]
@@ -917,6 +918,8 @@ class Config:
         ns, unknown_args = self._parser.parse_known_and_unknown_args(args)
         mode = getattr(ns, "assertmode", "plain")
         if mode == "rewrite":
+            import _pytest.assertion
+
             try:
                 hook = _pytest.assertion.install_importhook(self)
             except SystemError: