Implement static checks on issues and Python code

Start with some cleanup:

• scripts: Rename issue module to kernel_sec
• scripts: Refer consistently to 'branches' and not 'releases'
• scripts/kernel_sec: Make issue a real subclass of debian.deb822.Deb822
• scripts: Delete obsolete scripts

Fix warnings and errors:

• scripts: Fix flake8 warnings
• Fix notes syntax in CVE-2022-3114
• Adjust whitespace in some issues to match Deb822 output

Then implement the checks in CI:

• scripts: Add check-syntax script
• CI: Add GitLab CI configuration to run check-syntax and flake8