TravisCI: black needs Python 3.6+

Author: peterjc

.travis.yml

@@ -20,7 +20,8 @@ before_install:
  - "if grep '\t' *.py *.rst; then echo 'Tabs are bad, please use four spaces.'; false; fi"
  - "if grep -n -r '[[:blank:]]$' *.py *.rst; then echo 'Please remove trailing whitespace.'; false; fi"
  - pip install --upgrade pip setuptools
- - pip install flake8 flake8-black flake8-docstrings restructuredtext-lint
+ - pip install flake8 flake8-docstrings restructuredtext-lint
+ - "if [[ $TRAVIS_PYTHON_VERSION == '3.6' ]]; then pip install flake8-black; fi"
  - echo "Using restructuredtext-lint to check documentation"
  - restructuredtext-lint *.rst
  - echo "Using flake8 to check Python code"