Archive the repo

[JustinBeckwith]

Note: oauth2client is now deprecated. As such, it is unlikely that we will
address or respond to your issue. We recommend you use
google-auth and oauthlib.

So reading that message above, I think it's time to archive the repo. Any thoughts?

@broady @busunkim96 @crwilcox @tswast

[broady]

Sounds good in principle, I wonder who is using it, though?

[JustinBeckwith]

@crwilcox any way to figure that out?

[broady]

It looks like pretty close to zero issues have been filed in the last year, though, so archiving LGTM.