fixed compatability with py36

Author: brianmaissy

testing/test_pdb.py

@@ -181,7 +181,7 @@ def test_1():
         """)
         child = testdir.spawn_pytest("--pdb %s" % p1)
         child.expect("(Pdb)")
-        output = child.before
+        output = child.before.decode("utf8")
         child.sendeof()
         assert "captured stdout" not in output
         assert "captured stderr" not in output