GitHub Actions: Test on Python 3.14 release candidate 3

[cclauss]

Python v3.14 -- October 7th

• https://www.python.org/download/pre-releases
• https://www.python.org/downloads/release/python-3140rc3
• https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#allow-pre-releases

Pwntools Pull Request

Thanks for contributing to Pwntools! Take a moment to look at CONTRIBUTING.md to make sure you're familiar with Pwntools development.

Please provide a high-level explanation of what this pull request is for.

Testing

Pull Requests that introduce new code should try to add doctests for that code. See TESTING.md for more information.

Target Branch

Depending on what the PR is for, it needs to target a different branch.

You can always change the branch after you create the PR if it's against the wrong branch.

Branch	Type of PR
dev	New features, and enhancements
dev	Documentation fixes and new tests
stable	Bug fixes that affect the current stable branch
beta	Bug fixes that affect the current beta branch, but not stable
dev	Bug fixes for code that has never been released

Changelog

After creating your Pull Request, please add and push a commit that updates the changelog for the appropriate branch.
You can look at the existing changelog for examples of how to do this.