Update Python2 CI workflow to Ubuntu 20.04

GitHub no longer offers the Ubuntu 18.04 runner.
diff --git a/.github/workflows/python2_tests.yml b/.github/workflows/python2_tests.yml
index 779376a..aabb9ae 100644
--- a/.github/workflows/python2_tests.yml
+++ b/.github/workflows/python2_tests.yml
@@ -12,8 +12,8 @@
 
 jobs:
   test_python2:
-    name: Test with Python 2 on Ubuntu 18.04
-    runs-on: ubuntu-18.04
+    name: Test with Python 2 on Ubuntu 20.04
+    runs-on: ubuntu-20.04
 
     steps:
       - name: Check out code from GitHub
@@ -25,7 +25,7 @@
       - name: Install dependencies
         run: |
           sudo apt-get update
-          sudo apt-get install python-protobuf protobuf-compiler scons
+          sudo apt-get install python-protobuf protobuf-compiler scons python-is-python2
 
       - name: Run tests with Python 2
         run: |