[Key vault] Update type hinting key.py

[neoseurae12]

Description

update python type hinting in key.py

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• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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• Title of the pull request is clear and informative.
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Testing Guidelines

• Pull request includes test coverage for the included changes.

[github-actions]

Thank you for your contribution @neoseurae12! We will review the pull request and get back to you soon.

[mccoyp]

Hi @neoseurae12, thank you so much for the contribution! There's just a small change that we'd need, but otherwise this looks great 🙂

[mccoyp]

Thanks so much again! I'll go ahead and get this merged 🙂