pythonapp.yml request ubuntu-18.04, which GitHub no longer supports

[calebmsword]

Describe the bug
As of April 1, 2023, GitHub no longer supports ubuntu-18.04, yet the default workflow file requests ubuntu-18.04.

To Reproduce
Just follow the development guidelines for creating a new Kivy flower. Once you make any push to the repo, three of the jobs will hang with the message

Requested labels: ubuntu-18.04
Job defined at: <github_username>/<repo_name>/.github/workflows/pythonapp.yml@refs/heads/master
Waiting for a runner to pick up this job...

Expected behavior
The workflow should request a version of ubuntu that GitHub supports.