flake8: exclude venv and .tox folder

Excluding these two folders to avoid countless lint warnings
caused by dependencies in these two folders.

Change-Id: I2403b23f88cebb5941a4f9b5ac6cc34d107fd2f1
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/382837
Commit-Queue: Daniel Kutik <daniel.kutik@lavawerk.com>
Tested-by: Daniel Kutik <daniel.kutik@lavawerk.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

.flake8

@@ -11,3 +11,6 @@ extend-ignore =
     E402,
     # E731: do not assign a lambda expression, use a def
     E731,
+exclude =
+    venv,
+    .tox,