python3.7 raises flake8 warning

[sobolevn]

It is a bug in flake8:

• FutureWarning: Possible nested set for EXTRANEOUS_WHITESPACE_REGEX PyCQA/pycodestyle#728

We currently allow python3.7 build to fail.

=============================== warnings summary ===============================
tests/test_visitors/test_wrong_class/test_base_class.py::FLAKE8
  /home/travis/virtualenv/python3.7.0/lib/python3.7/site-packages/pycodestyle.py:113: FutureWarning: Possible nested set at position 1
    EXTRANEOUS_WHITESPACE_REGEX = re.compile(r'[[({] | []}),;:]')
-- Docs: https://docs.pytest.org/en/latest/warnings.html
=================== 1514 passed, 1 warnings in 27.96 seconds ===================

[sobolevn]

We will just ignore this warning for now.