This makes it possible to PUT a project after attach_project_pictures()
has been called on it (which embeds the picture file documents).
This will be used in SVNman.
These changes mostly revolve around the change in ObjectId constructor
when running on Python 3.6. Where on 2.7 the constructor would accept
12- and 24-byte strings, now only 12-byte bytes and 24-character strings
are accepted. Good thing, but required some changes in our code.
Other changes include hashing of strings, which isn't supported, so they
are converted to bytes first, and sometimes converted back afterwards.
