build: find Python syntax errors in dependencies

As discussed in #30129 (comment), when we vendor in code, we own the Syntax Errors in that code.  This PR adds The `.flake8` config file at the root of this repo puts blinders on the linting of our dependencies so this test disables that file before linting.

Author: cclauss

.travis.yml

@@ -88,3 +88,13 @@ jobs:
         - if [ "${TRAVIS_PULL_REQUEST}" != "false" ]; then
             bash -x tools/lint-pr-commit-message.sh ${TRAVIS_PULL_REQUEST};
           fi
+
+    - name: "Find syntax errors in our Python dependencies"
+      language: python
+      python: 3.8
+      install:
+        - mv .flake8 disabled.flake8  # take the blinders off of flake8
+        - python3.8 -m pip install --upgrade pip
+        - python3.8 -m pip install flake8
+      script:
+        - flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics