ci: Add explicit permissions to CI workflows.

[nedseb]

Description

CodeQL alerts #1, #2, #3 report actions/missing-workflow-permissions on the three CI workflows. Workflows without explicit permissions default to the repository's default token permissions, which may be overly broad.

Tasks

• Add permissions: contents: read to tests.yml
• Add permissions: contents: read to check-commits.yml
• Add permissions: contents: read to python-linter.yml

Related

• PR ci: Add explicit permissions to CI workflows. #276