rosdep doesn't "see" Python 3 package

edit

i'm installing cartography,when i executing the command

rosdep install --from-paths src --ignore-src --rosdistro=${ROS_DISTRO} -y

in https://google-cartographer-ros.readt... always turn to executing command

sudo -H apt-get install -y python-sphinx

however,i'm using python3 and the rosdep version in my ubuntu20.04LTS is python3-rosdep,why it will turn to install python-sphinx?i think the right sphinx version for this is python3-sphinx and i apt-get install it and it does be installed ,then it still reports the same error.therefore i am stuck in the above commands.

here are the errors:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package python-sphinx is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
  sphinx-doc sphinx-common

E: Package 'python-sphinx' has no installation candidate
ERROR: the following rosdeps failed to install
  apt: command [sudo -H apt-get install -y python-sphinx] failed