Adding mypy.

[jvachier]

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• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (code improvements without changing functionality)

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• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works.
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