fix pytest deprecation error

[skeptycal]

deprecation warning reported by pytest during testing: direct contruction is deprecated as of pytest 5.4 and will be removed in the future. Use of the constructor [Node].from_parent is recommended.

$ py setup test

conftest.py:14
  .../docopt/conftest.py:14: PytestDeprecationWarning: direct construction of DocoptTestFile has been deprecated, please use DocoptTestFile.from_parent
    return DocoptTestFile(path, parent)

conftest.py:44: 175 tests with warnings
  .../docopt/conftest.py:44: PytestDeprecationWarning: direct construction of DocoptTestItem has been deprecated, please use

DocoptTestItem.from_parent
    yield DocoptTestItem("%s(%d)" % (name, index), self, doc, case)

-- Docs: https://docs.pytest.org/en/latest/warnings.html