fix: remove tests from distributed packages

[louison]

Solves #90

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Note

Low Risk
Low risk packaging-only change that alters what gets included in the built wheel/sdist. Main risk is inadvertently excluding non-test modules if they live under tests or rely on it at runtime.

Overview
Updates setup.py to exclude the tests package from find_packages, preventing test code from being shipped in the distributed customerio package.

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