Add back sdist generation and deployment.

[jsirois]

This implements a workaround for pex lack of support for manylinux1
as outlined in #4956.

[kwlzn]

one potential alternative here would be to make a copy of the manylinux1_x86_64 bdist to with the platform filename subbed to linux_x86_64 and then publish that alongside the manylinux wheel. it should "just work" for the pex resolver, as manylinux1-x86_64 is essentially just a targeted subset of linux-x86_64.

I also have a branch adding manylinux support to pex that's 90% or more of the way there but I just haven't had time to devote to it in the face of other priorities.