Add issue number and removal TODO

Author: torext

Diff for: pandas/tests/plotting/frame/test_frame.py

@@ -2489,6 +2489,8 @@ def test_secondary_y(self, secondary_y):
 
     @pytest.mark.slow
     def test_plot_no_warning(self):
+        # GH 55138
+        # TODO(3.0): this can be removed once Period[B] deprecation is enforced
         df = tm.makeTimeDataFrame()
         with tm.assert_produces_warning(False):
             _ = df.plot()

Diff for: pandas/tests/plotting/test_series.py

@@ -976,5 +976,7 @@ def test_series_none_color(self):
 
     @pytest.mark.slow
     def test_plot_no_warning(self, ts):
+        # GH 55138
+        # TODO(3.0): this can be removed once Period[B] deprecation is enforced
         with tm.assert_produces_warning(False):
             _ = ts.plot()