tests: check for early version of 3.15

[boidolr]

No description provided.

[Copilot]

Pull request overview

Expands the project’s CI/test matrix to exercise an early Python 3.15 interpreter, ensuring compatibility issues are detected sooner.

Changes:

• Add a py315 tox factor to the default tox envlist.
• Teach tox-gh-actions to map GitHub Actions’ 3.15 to py315.
• Add Python 3.15 to the GitHub Actions matrix and enable prerelease interpreter resolution.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tox.ini	Adds py315 to tox envs and maps GitHub Actions 3.15 to py315.
.github/workflows/main.yaml	Adds 3.15 to CI matrix and enables prerelease Python installs for that job.

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