Turn coroutine to async def

[danixeee]

Description (e.g. "Related to ...", etc.)

Turn coroutine to async def to resolve deprecation warning:

"@coroutine" decorator is deprecated since Python 3.8, use "async def" instead.

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly
• CONTRIBUTORS.md was updated, as appropriate
• Changelog has been updated, as needed (see CHANGELOG.md)