TST: Use datapath fixture (#30756)

This was failing the wheel build.

https://travis-ci.org/MacPython/pandas-wheels/jobs/633451994

Author: TomAugspurger

pandas/tests/io/test_pickle.py

@@ -381,10 +381,10 @@ def test_read(self, protocol, get_random_path):
             tm.assert_frame_equal(df, df2)
 
 
-def test_unicode_decode_error():
+def test_unicode_decode_error(datapath):
     # pickle file written with py27, should be readable without raising
     #  UnicodeDecodeError, see GH#28645
-    path = os.path.join(os.path.dirname(__file__), "data", "pickle", "test_py27.pkl")
+    path = datapath("io", "data", "pickle", "test_py27.pkl")
     df = pd.read_pickle(path)
 
     # just test the columns are correct since the values are random