Bump version to 3.1.0

[hownowstephen]

Summary

• Bump __version__ from 3.0.0 to 3.1.0
• Drop changelog in favour of github releases

Release

After merge, create a GitHub release tagged v3.1.0 to trigger publish workflow.

Note: the v3.1.0 tag already exists at HEAD of main — delete and re-tag after this merges, or create the release from the merge commit.

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Note

Low Risk
Low risk release-metadata update: bumps the package version and adjusts project metadata without touching runtime code paths.

Overview
Bumps the library version from 3.0.0 to 3.1.0 via customerio/__version__.py.

Removes the in-repo CHANGELOG.md and updates pyproject.toml project metadata to link to GitHub Releases instead of a changelog file.

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