Include flake8-print plugin

This alerts us of stray print statements, which should never occur
because this is a non-TUI application.

.github/workflows/main.yaml

@@ -45,7 +45,7 @@ jobs:
       - name: Install
         run: |
           set -euo pipefail
-          pip install flake8
+          pip install flake8 flake8-print
       - name: Linting
         run: |
           python -m flake8 snikket_web