"The RNG is seeded from os.urandom, which as of python 3.5 uses the potentially blocking getrandom() call."

Hum ok, so your issue is specific to Linux.

"This causes problems e.g. on our build VMs that don't have true entropy, so getrandom() blocks forever"

Hum, the problem was already fixed some months/years ago: you must attach a RNG virtio device to your VM. Python is just one example, a lot of applications need entropy.

"A possible workaround is to monkeypatch os.urandom (in this particular case, to return a string of zeroes and remove randomness entirely to get reproducible builds)"

An unsafe *workaround* is to install haveged, a daemon generating entropy using the CPU.