Support reddit-decider 1.18 on Python 3.9+

[drfranks3]

Summary

• support reddit-decider 1.18.x on Python 3.9 and newer
• retain the existing Decider 1.15.x range on Python 3.8
• test both Decider 1.15.0 and 1.18.1 in CI
• verify that a manifest containing $properties preserves ordinary experiment evaluation

Python 3.8 remains on Decider 1.15.x because the newer Decider dependency stack requires Python 3.9 or newer.

Rollout

The first package publication will be 1.11.0rc1 so consumers must opt in. A stable 1.11.0 release will follow only after the prerelease has been exercised by the pilot service.

Verification

• Decider 1.15.0: 203 passed, 19 skipped
• Decider 1.18.1: 204 passed, 18 skipped
• Python 3.8 release-toolchain wheel and sdist build
• twine check for both distributions
• wheel metadata verified for both conditional Decider ranges

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