depot_tools master->main transition

[Evan Green]

I lost about a day trying to work through the Simple Chrome workflow
for the first time and banging my head over why 'gclient runhooks'
kept failing with this error:

Could not find a version that satisfies the requirement
setuptools>=40.3.0 (from google_api_core==1.25.1->-r
/tmp/vpython_bootstrap394355674/requirements.txt (line 16))

Eventually I traced it to the fact that my original ~/depot_tools
checkout outside the chroot stopped updating in 2020. It's been
diligently tracking origin/master, but we all transitioned to
origin/main.

I probably missed an announcement email, or it didn't occur to me that
I needed to do anything since chroot magic did most of the transition
seamlessly for me.

I filed a bug at b/213235522 to discuss whether there's something we
could do to the master branch (eg delete or move all the files, or
stamp a warning in the binaries) to save future people from losing
time here. But I thought I'd still send this as an email in case
anyone else is struggling.

-Evan