Well, it is a known issuse.
We can see the following report:https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1825655
建议的解决方法是删除/usr/bin中的python二进制文件，然后创建符号链接并执行升级:
1. sudo rm /usr/bin/python
2. sudo ln -s /usr/bin/python2.7 /usr/bin/python
3. do-release-upgrade
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