Certain dependencies are only needed on old python versions,
e.g. backports of stdlib functionality.
We should definitely not use newer python when running freeze_packages.sh
than what we bundle in the binaries. Perhaps it is prudent to use the
min python version that we support (which is atm older than what we bundle).
error was:
In --require-hashes mode, all requirements must have their versions pinned with ==. These do not:
setuptools from 04a0b689d3cc121c90bfd85a1a8c2e/setuptools-39.0.1-py2.py3-none-any.whl#md5=ca299c7acd13a72e1171a3697f2b99bc (from protobuf==3.5.2.post1->-r ./contrib/deterministic-build/requirements.txt (line 21))
This is used to make sure we also freeze versions for
packages that will only be used on Windows or OS X, while
the freezing script is most likely only be run on Linux.