Regression in pymdownx.snippets reintroduces sibling-prefix path traversal bypass despite restrict_base_path

Summary pymdownx.snippets has a regression of the CVE-2023-32309 / GHSA-jh85-wwv9-24hv fix. With restrictbasepath: True the default, the current filename.startswithbase containment check does not enforce a directory boundary. As a result, a markdown snippet directive can read files from sibling...