Comment 1 for bug 1901384

melwitt thinks the reason behind this is the "Parameterized generics cannot be used with class" error that shows up in the nova compute log in the log URL you gave:

http://lists.openstack.org/pipermail/openstack-discuss/2020-October/018289.html

She seems to have isolated the problem to an incompatibility between python 3.6 and the version of libvirt-python currently allowed in upper-constraints:

http://lists.openstack.org/pipermail/openstack-discuss/2020-October/018287.html

Also, melwitt has a patch up to change the u-c, but there seems to be some controversy:

https://review.opendev.org/#/c/759552/