Merge pull request #14108 from 0xc0170/travis_remove_py3.5

travis: set attrs to 19.1.0, fixes python 3.5 issue

Author: 0xc0170

.travis.yml

@@ -229,6 +229,7 @@ matrix:
         - |-
           tr -d ' ' >> requirements.txt <<< "
           mock==2.0.0
+          attrs==19.1.0
           pytest==3.3.0
           pylint>=1.9,<2
           hypothesis>=3,<4