feat: add py.typed

[Goldziher]

Description of the change

Add py.typed file to package

Motivation

Without the py.typed, the library is considered untyped by type-checkers such as MyPy.

Type of change

Other

Checklist

• [x] I have performed a self-review of my code.
• [x] I have added detailed comments to my code where applicable.
• [x] I have verified that my change does not break existing code.
• [x] My PR is based on the latest changes of the main branch (if unsure, please run git pull --rebase upstream main).
• [x] I am familiar with the Google Style Guide for the language I have coded in.
• [x] I have read through the Contributing Guide and signed the Contributor License Agreement.

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