Travis CI: Use flake8 to find Python syntax errors and undefined names (#5467)

.travis.yml

@@ -1,4 +1,3 @@
-sudo: true
 dist: xenial
 language: python
 python:
@@ -29,7 +28,6 @@ jobs:
   include:
     - stage: binary builds
       name: "Windows build"
-      sudo: true
       language: c
       python: false
       env:
@@ -72,7 +70,6 @@ jobs:
       after_script: ls -lah dist && md5 dist/*
       after_success: true
     - name: "AppImage build"
-      sudo: true
       language: c
       python: false
       services:
@@ -82,6 +79,10 @@ jobs:
       script:
         - sudo docker run --name electrum-appimage-builder-cont -v $PWD:/opt/electrum --rm --workdir /opt/electrum/contrib/build-linux/appimage electrum-appimage-builder-img ./build.sh
       after_success: true
+    - name: "Flake8 tests"
+      language: python
+      install: pip install flake8
+      script: flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
     - stage: release check
       install:
           - git fetch --all --tags