fixed syntax

TomAugspurger · TomAugspurger · commit b7f0198af65d · 2018-09-16T14:50:00.000-05:00
diff --git a/pandas/tests/groupby/aggregate/test_cython.py b/pandas/tests/groupby/aggregate/test_cython.py
@@ -29,7 +29,7 @@
                  # ignore mean of empty slice
                  # and all-NaN
                  marks=[pytest.mark.filterwarnings(
-                     "ignore:RuntimeWarning"
+                     "ignore::RuntimeWarning"
                  )]),
     'prod',
     'min',
