Problem: Docstrings were handled differently on 3.7.0b4

This caused an increased complexity number on 3.7.0b4 when
docstring was used in the expr_as_statement test.

Solution:

While this change in Python will most likely get reverted
according to [1], we decided to use a non-docstring expression
in the test to avoid this problem in the future if docstring
handling ever gets changed again.

Fixes https://github.com/PyCQA/mccabe/issues/61

[1] https://github.com/PyCQA/mccabe/issues/61#issuecomment-393137462