fix: blacksheep example unittest error (#1534)

[waketzheng]

Description

Motivation and Context

The document of blacksheep suggests to use pytest_asyncio for async test, so I add it to dev group, and change _tests.py to be in accordance with blacksheep's demo.

How Has This Been Tested?

make test
cd examples/blacksheep && PYTHONPATH=. pytest _tests.py

Checklist:

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have added the changelog accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.