CI: update Ubuntu and Python in GitHub workflow

.github/workflows/ci.yaml

@@ -7,12 +7,12 @@ defaults:
 
 jobs:
   lint-python:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
         with:
-          python-version: '3.8'
+          python-version: '3.10'
       - run: |
           pip install "$(grep -E '^flake8' django/requirements-test.txt)"
           pip install "$(grep -E '^mypy' django/requirements-test.txt)"
@@ -21,7 +21,7 @@ jobs:
       - run: mypy django/
 
   lint-javascript:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-node@v2-beta
@@ -34,12 +34,12 @@ jobs:
         name: Ensure that docs are valid
 
   test-backend:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-22.04
     strategy:
       fail-fast: false
       matrix:
         # For PyPy use 'pypy-3.8'. It is disabled by default due to long build times.
-        python-version: ['3.8', '3.9']
+        python-version: ['3.8', '3.9', '3.10']
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2
@@ -66,7 +66,7 @@ jobs:
         name: Run tests
 
   test-frontend:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-22.04
     strategy:
       fail-fast: false
       matrix: