Fix type hints in WebapiAsync [2.3-develop]

[mhauri]

Description

This PR introduces no backward incompatibilities, it only fixes the type
hints to make the code more readable and improve compatibility with static
code analysis tools.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

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[magento-engcom-team]

Hi @mhauri. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.3.0 release.