Stable packages sit together while one testing package is raised above them with a visible version gap.

Question: I upgraded one package from Debian testing/unstable on my Debian stable and now I'm having weird issues
1 mindebianaptaptitudestabletestingunstableupgradelibc6package-managementversion-mismatch

Answer

You partially upgraded from stable to testing or unstable… Now you are encountering issues because of major version upgrades.

sh 
libc6-dev : Depends: libc6 (= 2.36-9+deb12u10) but 2.41-6 is to be installed

You see the 2.36-9+deb12u10, debian 12.10 on that package whereas unstable and testing both run 2.41-6:

zsh 
$ apt-cache policy libc6-dev
libc6-dev:
  Installed: 2.41-6
  Candidate: 2.41-6
  Version table:
 *** 2.41-6 900
        900 https://deb.debian.org/debian unstable/main amd64 Packages
        500 https://deb.debian.org/debian testing/main amd64 Packages
        100 /var/lib/dpkg/status
     2.36-9+deb12u10 10
         10 https://deb.debian.org/debian stable/main amd64 Packages

You need to fully upgrade your machine to either unstable or testing to make sense of it again. The closer you are to a major Debian release the more likely you’ll want to upgrade to testing. Otherwise use unstable

zsh 
sudo aptitude update # get all the new info from the repos
sudo aptitude safe-upgrade # upgrade everything to either testing/unstable
sudo aptitude full-upgrade # do all the upgrades that need removing packages

The latter will probably give you some “what you want to do” questions. For python3, libc6 and the other ones in your post you’ll want to install the versions from testing.

To prevent automatic upgrades on the development version of Debian, you’ll want to run sudo dpkg-reconfigure unattended-upgrades (see unattended-upgrades).