The ebuild SRC_URI is for notebook-4.0.5.tar.gz, however the PyPi file is notebook-4.0.5.tgz. Modifying the ebuild allows emerge without problems.
commit 3db5c87d6b252b2a5ede3f7894f9ebde65093240 Author: Marius Brehler <marbre@linux.sungazer.de> Date: Fri Oct 2 16:27:55 2015 +0200 dev-python/notebook: Fix SRC_URI Package-Manager: portage-2.2.20.1