Merge bitcoin#14579: [0.17] travis: Pin flake8 version to 3.5.0

f9fc08c travis: Pin flake8 version to 3.5.0 (fanquake)

Pull request description:

  Pin flake8 version to 3.5.0 to fix the linters.

Tree-SHA512: 4f06d0aacbdfbbc6d090790aeea5185f2a61259381e586f4015bc0f8c0d0c8fec6156343a9587b8605bc776b39c080bd6584e6e9b685eaba3d90a58c726ac469

.travis.yml

@@ -148,7 +148,7 @@ jobs:
       language: python
       python: '3.6'
       install:
-        - travis_retry pip install flake8
+        - travis_retry pip install flake8==3.5.0
       before_script:
         - git fetch --unshallow
       script: