run_tests: Check flake8

This also gets enforced in CQ.

Bug: b/267675342
Change-Id: I8ffcc5d583275072fd61ae65ae4214b36bfa59f3
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/366799
Tested-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

tox.ini

@@ -28,6 +28,7 @@ python =
 [testenv]
 deps =
     black
+    flake8
     pytest
     pytest-timeout
 commands = {envpython} run_tests {posargs}