Luckily there are still some mirrors on the internet which have this package. You can run this command to install it; afterwards installing indico 2.0 should work:
pip install https://files.pythonhosted.org/packages/ee/a3/ca8d9d4255e71d12c2f88523f431b88868ec73c2fdeae7eca8e64ae0908e/pyatom-1.4.tar.gz'