presubmits: add types to check-presubmit.py.

Since presubmits handle py3 gracefully now && we're starting to add
types in many places, it seems worth it to add these for better
understandability in the future.

No functional change is intended.

BUG=chromium:1100659
TEST=Ran the presubmits.

Change-Id: If4134b4d10b0f3692a52803e12c8e503b4574be5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/2274727
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Tested-by: George Burgess <gbiv@chromium.org>
1 file changed