Let xfail work on non-python Items

For some reason xfail was only implemented on non-python Item
instances.  This removes this guard which means plugins creating new
items can raise pytest.xfail.Exception and it will work as expected.

_pytest/skipping.py

@@ -58,6 +58,7 @@ class MarkEvaluator:
     @property
     def holder(self):
         return self.item.keywords.get(self.name, None)
+
     def __bool__(self):
         return bool(self.holder)
     __nonzero__ = __bool__
@@ -136,8 +137,6 @@ class MarkEvaluator:
 
 @pytest.mark.tryfirst
 def pytest_runtest_setup(item):
-    if not isinstance(item, pytest.Function):
-        return
     evalskip = MarkEvaluator(item, 'skipif')
     if evalskip.istrue():
         pytest.skip(evalskip.getexplanation())
@@ -155,8 +154,6 @@ def check_xfail_no_run(item):
                 pytest.xfail("[NOTRUN] " + evalxfail.getexplanation())
 
 def pytest_runtest_makereport(__multicall__, item, call):
-    if not isinstance(item, pytest.Function):
-        return
     # unitttest special case, see setting of _unexpectedsuccess
     if hasattr(item, '_unexpectedsuccess'):
         rep = __multicall__.execute()

testing/test_skipping.py

@@ -678,3 +678,23 @@ class TestBooleanCondition:
             *True123*
             *1 xfail*
         """)
+
+
+def test_xfail_item(testdir):
+    # Ensure pytest.xfail works with non-Python Item
+    testdir.makeconftest("""
+        import pytest
+
+        class MyItem(pytest.Item):
+            nodeid = 'foo'
+            def runtest(self):
+                pytest.xfail("Expected Failure")
+
+        def pytest_collect_file(path, parent):
+            return MyItem("foo", parent)
+    """)
+    result = testdir.inline_run()
+    passed, skipped, failed = result.listoutcomes()
+    assert not failed
+    xfailed = [r for r in skipped if hasattr(r, 'wasxfail')]
+    assert xfailed