I checked the release notes for 17.10 and Python 3.6 is indeed the default.
You shouldn't need to downgrade it.
Can you check whether your graph-tool line in /etc/apt/sources.list is
pointing to "artful" and not another release? i.e.:
deb http://downloads.skewed.de/apt/*artful* *artful* universe
If this is correct, there seems to be an issue with how graph-tool was
compiled for 17.10, in which case you'd have to compile it yourself or wait
for the package to be fixed.
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