Fix error message with unregistered markers

src/_pytest/mark/structures.py

@@ -306,7 +306,7 @@ class MarkGenerator(object):
             # then it really is time to issue a warning or an error.
             if name not in self._markers:
                 if self._config.option.strict:
-                    fail("{!r} not a registered marker".format(name), pytrace=False)
+                    fail("{!r} is not a registered marker".format(name), pytrace=False)
                 else:
                     warnings.warn(
                         "Unknown pytest.mark.%s - is this a typo?  You can register "

testing/test_mark.py

@@ -204,7 +204,7 @@ def test_strict_prohibits_unregistered_markers(testdir):
     )
     result = testdir.runpytest("--strict")
     assert result.ret != 0
-    result.stdout.fnmatch_lines(["'unregisteredmark' not a registered marker"])
+    result.stdout.fnmatch_lines(["'unregisteredmark' is not a registered marker"])
 
 
 @pytest.mark.parametrize(