Revert "Revert "XXX travis-ci's py3.6 is not up to date""

This reverts commit bcd2c90.

diofant/core/tests/test_cache.py

@@ -1,3 +1,5 @@
+import sys
+
 import pytest
 
 from diofant.core.symbol import Symbol
@@ -27,6 +29,8 @@ def test_cacheit():
     assert _identity(1) == 1
 
 
+@pytest.mark.skipif(sys.version_info >= (3, 6),
+                    reason="https://github.com/pytest-dev/pytest/issues/1957")
 def test_print_cache(capfd):
     clear_cache()
     wrapped = _identity.__wrapped__