remove pytest_namespace from _pytest.skipping

pytest.py

@@ -18,6 +18,7 @@ __all__ = [
     'yield_fixture',
     'fail',
     'skip',
+    'xfail',
     'importorskip',
     'exit',
     'mark',
@@ -43,6 +44,7 @@ from _pytest.debugging import pytestPDB as __pytestPDB
 from _pytest.recwarn import warns, deprecated_call
 from _pytest.runner import fail, skip, importorskip, exit
 from _pytest.mark import MARK_GEN as mark
+from _pytest.skipping import xfail
 set_trace = __pytestPDB.set_trace
 
 _preloadplugins() # to populate pytest.* namespace so help(pytest) works