Classify as Stable on PyPI

[adamchainz]

Use the produciton/stable PyPI classifier, so people don't think this package is beta-quality software.

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [x] Feature
• [ ] Code style update
• [ ] Refactor
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

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