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=. poetry run pytest _tests.py

Checklist:

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

[abondar]

We should also rebase this to actual, but may be after merging #1540

[coveralls]

Pull Request Test Coverage Report for Build 8863610731

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 34 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.03%) to 87.934%

---

Files with Coverage Reduction	New Missed Lines	%
tortoise/models.py	7	97.24%
tortoise/queryset.py	27	94.58%

Totals
Change from base Build 8858003658:	0.03%
Covered Lines:	5771
Relevant Lines:	6467

---

💛 - Coveralls

[waketzheng]

@abondar done.