[bikeshed] Bikeshed is now on Python 3; update instructions enclosed

A bit late of where I intended (since Python 2 was EOL'd on Jan 1),
but Bikeshed is now a Python 3 project. If you clone fresh from
master, it will no longer work on Python 2 (and in fact requires
Python 3.7).

I'm writing some updated install instructions right now, but if you
have some CI that uses Bikeshed, there's a quick fix you can deploy
that'll keep you going on the last Py2 commit. This won't ever update,
so you'll want to fix it properly (and I'll follow up with another
email to this list when I have instructions for it), but for now, just
go into the file that tells your CI what to do, and replace

git clone https://github.com/tabatkins/bikeshed.git

with

git clone https://github.com/tabatkins/bikeshed.git -b v0.9

For example, <https://github.com/w3c/w3process/commit/03020e861c6f261acaab862b2d7af45786f97ce4>
is Florian updating their Travis config.

~TJ