Add flake8 to CI, too

.flake8

@@ -4,5 +4,5 @@
 [flake8]
 ignore = E266, E501
 max-line-length = 80
-max-complexity = 12
+max-complexity = 15
 select = B,C,E,F,W,T4,B9

.travis.yml

@@ -2,11 +2,12 @@ sudo: false
 language: python
 cache: pip
 before_script:
-- pip install mypy
+- pip install flake8 flake8-bugbear mypy
 - pip install -e .
 script:
 - python setup.py test
 - if [[ $TRAVIS_PYTHON_VERSION == '3.6' ]]; then mypy black.py tests/test_black.py; fi
+- if [[ $TRAVIS_PYTHON_VERSION == '3.6-dev' ]]; then flake8 black.py tests/test_black.py; fi
 notifications:
   on_success: change
   on_failure: always

black.py

@@ -807,7 +807,7 @@ def __attrs_post_init__(self) -> None:
 ALWAYS_NO_SPACE = CLOSING_BRACKETS | {token.COMMA, token.COLON, STANDALONE_COMMENT}
 
 
-def whitespace(leaf: Leaf) -> str:
+def whitespace(leaf: Leaf) -> str:  # noqa C901
     """Return whitespace prefix if needed for the given `leaf`."""
     NO = ''
     SPACE = ' '