Fix CI job to include tomli in zipapp

[sgryjp]

• I have added an entry to docs/changelog.md

Summary of changes

This PR changes the CI job for creating zipapp file (pipx.pyz) to use Python 3.8. Doing so makes shiv include tomli and the zipapp can be executed with Python 3.10 or earlier.

This should closes #1142.

Test plan

Tested by running python pipx.pyz reinstall black and then confirm the installed black works by running black --version, for Python 3.8 through 3.12:

mac:~% for VER in 3.8 3.9 3.10 3.11 3.12; do python$VER pipx.pyz reinstall black; black --version; done
uninstalled black! ✨ 🌟 ✨
  installed package black 23.12.1, installed using Python 3.8.18
  These apps are now globally available
    - black
    - blackd
done! ✨ 🌟 ✨
black, 23.12.1 (compiled: yes)
Python (CPython) 3.8.18
uninstalled black! ✨ 🌟 ✨
  installed package black 23.12.1, installed using Python 3.9.18
  These apps are now globally available
    - black
    - blackd
done! ✨ 🌟 ✨
black, 23.12.1 (compiled: yes)
Python (CPython) 3.9.18
uninstalled black! ✨ 🌟 ✨
  installed package black 23.12.1, installed using Python 3.10.13
  These apps are now globally available
    - black
    - blackd
done! ✨ 🌟 ✨
black, 23.12.1 (compiled: yes)
Python (CPython) 3.10.13
uninstalled black! ✨ 🌟 ✨
  installed package black 23.12.1, installed using Python 3.11.6
  These apps are now globally available
    - black
    - blackd
done! ✨ 🌟 ✨
black, 23.12.1 (compiled: yes)
Python (CPython) 3.11.6
uninstalled black! ✨ 🌟 ✨
  installed package black 23.12.1, installed using Python 3.12.1
  These apps are now globally available
    - black
    - blackd
done! ✨ 🌟 ✨
black, 23.12.1 (compiled: yes)
Python (CPython) 3.12.1

[dukecat0]

Thanks!