rust_tools: fix mypy type complaints

This fixes all type complaints issued by
`PYTHONPATH=.. python3 -m mypy --follow-imports=silent *.py` in this
directory.

BUG=b:314767343
TEST=mypy

Change-Id: Ic376bc75d601987e30ed30db446c2842b3fe479f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/5084477
Tested-by: George Burgess <gbiv@chromium.org>
Reviewed-by: Bob Haarman <inglorion@chromium.org>
Commit-Queue: George Burgess <gbiv@chromium.org>
3 files changed