Potential fix for code scanning alert no. 1: Workflow does not contain permissions

Autofix for https://github.com/railsware/mailtrap-python/security/code-scanning/1

Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>

Author: mklocek

.github/workflows/main.yml

@@ -2,6 +2,9 @@ name: Tests and checks
 
 on: push
 
+permissions:
+  contents: read
+
 jobs:
   check:
     name: Test python${{ matrix.python }} on ${{ matrix.os }}