Update pytest-asyncio requirement from ^0.16.0 to ^0.18.3

Updates the requirements on [pytest-asyncio](https://github.com/pytest-dev/pytest-asyncio) to permit the latest version.
- [Release notes](https://github.com/pytest-dev/pytest-asyncio/releases)
- [Changelog](https://github.com/pytest-dev/pytest-asyncio/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/pytest-dev/pytest-asyncio/compare/v0.16.0...v0.18.3)

---
updated-dependencies:
- dependency-name: pytest-asyncio
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

pyproject.toml

@@ -28,7 +28,7 @@ toml = "^0.10.2"
 sphinx-rtd-theme = "^1.0.0"
 sphinx-automodapi = "^0.13"
 pytest-cov = "^3.0.0"
-pytest-asyncio = "^0.16.0"
+pytest-asyncio = "^0.18.3"
 openapi-parser = "^0.2.6"
 autopep8 = "^1.6.0"
 prance = "^0.21.8"