Update Python version to 3.15 in configuration and tests

[hakandilek]

Summary

• Python 3.15 is supported in configuration and validation toolset.
• Library is tested against Python 3.15
• Dependency tree is tested against Python 3.15, Deepdiff dependency introducing a regression.

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DeepDiff regression

Python 3.15 testing is currently blocked by an upstream dependency constraint in DeepDiff. We will wait for an upstream fix rather than introduce a local workaround:

• DeepDiff 9.1.0 requires cachebox>=5.2,<6.
• Cachebox 5.2.3 uses PyO3 0.27.1, which supports Python only through 3.14.
• On Python 3.15, dependency installation falls back to building Cachebox from source, and PyO3 rejects the interpreter version.
• The failure occurs before any tests run; it does not establish an incompatibility in this library.

Cachebox 6.2.8 already provides Python 3.15 wheels, including macOS ARM64. The remaining blocker is DeepDiff's exclusion of Cachebox 6.

Decision

• Do not downgrade DeepDiff, force an incompatible dependency resolution, or introduce compiler workarounds.
• Wait for a DeepDiff release that permits Cachebox 6.
  • Issue #601 — Migrate to cachebox 6: Open; explicitly tracks Python 3.15 support.
  • PR #623 — Allow CacheBox 6 for Windows ARM64 support: Open; widens the requirement to cachebox>=5.2,<7, which would permit 6.2.8.
• Python 3.15 validation remains blocked until that dependency update.

Follow-up

1. Upgrade DeepDiff in pyproject.toml and refresh poetry.lock.
2. Validate Python 3.15 with poetry run tox run -e 3.15.
3. Run the full validation matrix with poetry run tox run.