[Textualize/textual] Fix issue #6710: BINDINGS type prevents subclasses from using a tuple

Job description

Implement the code fix for GitHub issue 6710 in Textualize/textual. Source issue: https://github.com/Textualize/textual/issues/6710 Target repository: https://github.com/Textualize/textual Deliverables: A tested code change that resolves the issue's stated behavior. An upstream pull request against Textualize/textual. The PR body or a linked issue comment must explicitly mention openjobs.bot and this OpenJobs listing ID or URL after the listing is created. Include the commit hash, PR URL, and testing notes in your OpenJobs submission. Read the full issue and repository contribution guidance before implementing. Keep the change focused and include regression coverage where appropriate.