cannot upgrade with mirrors.bit.edu.cn

[jingkaimori]

I ran following command to upgrade my dist,but it fails.It seems like the dependency of python and qt is broken.
```
sudo apt-get update
sudo apt-get dist-upgrade
sudo do-release-upgrade desktop
```
I installed steam on my system.

ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: ubuntu-release-upgrader-core 1:19.10.15.4
ProcVersionSignature: Ubuntu 5.3.0-64.58-generic 5.3.18
Uname: Linux 5.3.0-64-generic x86_64
ApportVersion: 2.20.11-0ubuntu8.9
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: KDE
Date: Fri Sep 18 12:11:02 2020
InstallationDate: Installed on 2019-11-01 (321 days ago)
InstallationMedia: Ubuntu 19.04 "Disco Dingo" - Release amd64 (20190416)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to eoan on 2020-09-18 (0 days ago)

[Brian Murray (brian-murray)]

The upgrade failed to calculate as you are using a non-official mirror of the Ubuntu archive.

2020-09-18 12:09:12,978 DEBUG examining: 'deb http://mirrors.bit.edu.cn/ubuntu/ eoan main restricted universe multiverse'
2020-09-18 12:09:12,980 DEBUG entry '# deb http://mirrors.bit.edu.cn/ubuntu/ focal main restricted universe multiverse # 已禁止升级到 focal' was disabled (unknown mirror)
2020-09-18 12:09:12,980 DEBUG examining: 'deb-src http://mirrors.bit.edu.cn/ubuntu/ eoan main restricted universe multiverse'
2020-09-18 12:09:12,981 DEBUG entry '# deb-src http://mirrors.bit.edu.cn/ubuntu/ focal main restricted universe multiverse # 已禁止升级到 focal' was disabled (unknown mirror)
2020-09-18 12:09:12,981 DEBUG examining: 'deb http://mirrors.bit.edu.cn/ubuntu/ eoan-security main restricted universe multiverse'
2020-09-18 12:09:12,982 DEBUG entry '# deb http://mirrors.bit.edu.cn/ubuntu/ eoan-security main restricted universe multiverse # 已禁止升级到
 focal' was disabled (unknown mirror)

Please try switching mirrors of the Ubuntu archive and try the upgrade again.

[Launchpad Janitor (janitor)]

[Expired for ubuntu-release-upgrader (Ubuntu) because there has been no activity for 60 days.]