TST: xfails unary neg of timeperiod

Current arithmetic tests for timeperiod dtype uses unary negative to
test expected output. Whether to allow this and how is currently under
discussion (see GH 19297). Until consensus is reached, test will be
marked as expected to fail.

Author: deniederhut

pandas/tests/frame/test_arithmetic.py

@@ -1,13 +1,16 @@
 # -*- coding: utf-8 -*-
 
 import numpy as np
+import pytest
 
 import pandas as pd
 import pandas.util.testing as tm
 
 
 class TestPeriodFrameArithmetic(object):
 
+    @pytest.mark.xfail(reason="Unary negative is currently not permitted "
+                              "for objects of type TimePeriod, see GH 19297")
     def test_ops_frame_period(self):
         # GH 13043
         df = pd.DataFrame({'A': [pd.Period('2015-01', freq='M'),