Failing test cases for string formating in newer Python versions

[abonie]

Current Behavior

Few tests still fail for newer Python versions (3.5+), namely:
tests.datatypes.test_str.FormatTests.test_basic in 3.5 and tests.datatypes.test_str.FormatTests.test_basic and tests.datatypes.test_str.NewStyleFormatTests.test_groupings in 3.6
It seems like something else than just a difference in error messages

Steps to reproduce

1. Run mentioned test cases from tests/datatypes/test_str.py

Your Environment

• Python Version (list the specific version number)
  3.5.3 and 3.6.0