This is because, in reality, graph-tool is a C++ library wrapped in Python, and it has many C++ dependencies such as Boost, CGAL and expat, which are not installable via Python-only package management systems such as pip. Because the module lives between the C++ and Python worlds, its installation is done more like a C++ library rather than a typical python module. https://git.skewed.de/count0/graph-tool/wikis/installation-instructions#native-installation
add public key to verify the packages
sudo apt-key adv --keyserver pgp.skewed.de --recv-key 612DEFB798507F25