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.

fixup: allow_failures until dependencies pass

PR-URL: #30143
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>

.travis.yml

@@ -91,3 +91,15 @@ 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
+  allow_failures:  # TODO (cclauss): remove this when dependencies are clean
+    - name: "Find syntax errors in our Python dependencies"