Run check_go_mod_tidy.py with python3 in presubmit
Run check_go_mod_tidy.py with python3 in presubmit

Bug: 1313804
Change-Id: Iee0891a16b20d10f7ad14091de7451997149a810
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3707088
Auto-Submit: Joey Scarr <e609d4c46132d2dea94acf5ac2b9f229ffc1f245@google.com>
Commit-Queue: Joey Scarr <e609d4c46132d2dea94acf5ac2b9f229ffc1f245@google.com>
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go