Fix build (#5086)

Run the mypy test suite using Ubuntu 18.04. Ubuntu 20.04 doesn't have a package for virtualenv on Python 2.

Also, curses.color_pair() now has a named argument.

.github/workflows/tests.yml

@@ -79,7 +79,8 @@ jobs:
 
   mypy-test-suite:
     name: Run the mypy test suite
-    runs-on: ubuntu-latest
+    # Ubuntu 20.04 doesn't have the Python 2 venv module.
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v2