You can uninstall or removes an installed astro-python3 package itself from Debian 9 (Stretch) through the terminal,
$ sudo apt-get remove astro-python3
If you would like to remove astro-python3 and it's dependent packages which are no longer needed from Ubuntu,
$ sudo apt-get remove --auto-remove astro-python3
If you use with purge options to astro-python3 package all the configuration and dependent packages will be removed.
$ sudo apt-get purge astro-python3
If you use purge options along with auto remove, will be removed everything regarding the package, It's really useful when you want to reinstall again.
$ sudo apt-get purge --auto-remove astro-python3