riscv64-port installs sagemath unsuccessfully.

[Larry Wei]

$ sudo apt-get install sagemath
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 sagemath : Depends: python3-sage (>= 9.5-4) but it is not installable
            Depends: python3-sage (< 9.5-4.1~) but it is not installable
            Recommends: sagemath-doc (= 9.5-4) but it is not going to be installed
            Recommends: sagemath-jupyter (= 9.5-4) but it is not going to be installed
            Recommends: sagetex but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

$ lsb_release -rd
Description: Ubuntu 22.04 LTS
Release: 22.04

$ apt-cache policy sagemath
sagemath:
  Installed: (none)
  Candidate: 9.5-4
  Version table:
     9.5-4 500
        500 https://mirrors.bfsu.edu.cn/ubuntu-ports jammy/universe riscv64 Packages

$ apt info python3-sage
Package: python3-sage
State: not a real package (virtual)
N: Can't select candidate version from package python3-sage as it has no candidate
N: Can't select versions from package 'python3-sage' as it is purely virtual
N: No packages found