[tlbuild] tl25 start

[Karl Berry]

I've switched the TL repo's trunk to target the pretest and am
committing a first set of autobuilt binaries. Those doing their own
commits, please do.

There's a new version of asy, 2.96, in the repo. Its new Python
requirement has defeated me on tug.org (thanks Python). Please don't
commit new asy binaries until this is resolved. I'm guessing there will
be a 2.97 pretty soon.

Thanks,
Karl