ubuntu-18.04 runners have been removed. Switch to ubuntu-latest

Author: misl6

.github/workflows/pythonapp.yml

@@ -3,7 +3,7 @@ on: [push, pull_request]
 
 jobs:
   lint_test:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v1
     - name: Set up Python 3.x
@@ -19,7 +19,7 @@ jobs:
         python3 -m flake8 . --count --show-source --statistics
 
   linux_test:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     env:
       DISPLAY: :99.0
     steps:
@@ -83,7 +83,7 @@ jobs:
         python -m pytest --cov=kivy_garden.$flower_name --cov-report term --cov-branch
 
   docs:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v1
     - name: Set up Python 3.x