Allow final CPython 3.15 in the blocking canary #136

Problem

Backtest's blocking Python 3.15 canary rejects a final CPython 3.15 runtime even when package installation and imports succeed. On PR #135's CI run, the Windows runner installed CPython 3.15.0 and completed dependency installation. The step named "Verify beta-or-later CPython 3.15 and package import" failed because its assertion permits only beta and candidate release levels.

Expected behavior

The canary should accept beta, release candidate, and final CPython 3.15 builds while still checking the Python minor version, numerical runtime, and Backtest import. This does not change the supported stable Python range or external comparison availability.

Reproduction and acceptance

• The existing workflow run supplies the real failing input: CPython 3.15.0 final on Windows, with the assertion failing after package imports.
• Update the verification so that a 3.15 final runtime reaches the test suite on Linux, macOS, and Windows.
• Re-run the blocking compatibility and ecosystem checks on the proposed fix. Preserve the 3.12-3.14 stable matrix and the documented 3.15 canary evidence.

AI assistance

Codex drafted this issue from the CI job log and inspected workflow. It will verify the proposed change through the workflow; human review of the public fix is pending.