ci: add permission to write coverage comments

.github/workflows/pythonpackage.yml

@@ -7,6 +7,14 @@ on: [ push, pull_request ]
 
 jobs:
   test:
+    permissions:
+      # Gives the action the necessary permissions for publishing new
+      # comments in pull requests.
+      pull-requests: write
+      # Gives the action the necessary permissions for pushing data to the
+      # python-coverage-comment-action branch, and for editing existing
+      # comments (to avoid publishing multiple comments in the same PR)
+      contents: write
 
     runs-on: ubuntu-latest
     strategy: