Replace item.get_marker with item.get_closest_marker as it was remove…

…d upstream as mentioned here: pytest-dev/pytest#4606 (comment)

Quoted here incase that link breaks in the future:
        "We have removed item.get_marker, users should use item.get_closest_marker now
        (it's a long history, this was not an arbitrary change). Indeed I suspect
        pytest-cov might need to be updated."

item.get_marker was removed in this PR pytest-dev/pytest#4564

testing/sdk_utils.py

@@ -111,7 +111,7 @@ def your_test_here(): ...
 
     In order for this annotation to take effect, this function must be called by a pytest_runtest_setup() hook.
     """
-    min_version_mark = item.get_marker("dcos_min_version")
+    min_version_mark = item.get_closest_marker("dcos_min_version")
     if min_version_mark:
         min_version = min_version_mark.args[0]
         message = "Feature only supported in DC/OS {} and up".format(min_version)