Hi there,
The section Debian installation on graph-tool download page [1] is not working. The reason of that is a mirror url [2] redirect from http to https. To solve this problem I suggest: You explain accurately every necessary steps to install graph-tool, including (before mirror update) a instruction to install apt-transport-https package or, another possible way to solve this, you simply remove the 'redirect' from the mirror page.
[1] https://graph-tool.skewed.de/download
[2] http://downloads.skewed.de/apt
Best regards,
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