Add a config file for the flake8 linter.

Part of my bigger auto{ninja,siso} refactoring.

R=brucedawson@chromium.org, jwata@google.com, tikuta@chromium.org

Bug: b/293657720
Change-Id: I2a7783b8675ecc7dac16a490a290b2edf4b9d64d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4924716
Reviewed-by: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Joanna Wang <jojwang@chromium.org>
Auto-Submit: Philipp Wollermann <philwo@chromium.org>

.flake8

@@ -0,0 +1,3 @@
+[flake8]
+max-line-length = 80
+