py3: use zlib module to compress the build proto stream
py3: use zlib module to compress the build proto stream

binary serializing returns bytes in py3 and it doesn't have encode
method.

R=iannucci, yuanjunh

Bug: 1227763
Change-Id: I52fcb3a983ba6ec0708f3d8abd7a34919e30afef
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/3051470
Auto-Submit: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/recipes-py,luci/recipes-py