Update pytest-asyncio requirement from ^0.19.0 to ^0.20.3 (#63)

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/v0.19.0...v0.20.3)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

pyproject.toml

@@ -28,7 +28,7 @@ toml = "^0.10.2"
 sphinx-rtd-theme = "^1.0.0"
 sphinx-automodapi = "^0.14"
 pytest-cov = "^4.0.0"
-pytest-asyncio = "^0.19.0"
+pytest-asyncio = "^0.20.3"
 openapi-parser = "^0.2.6"
 autopep8 = "^1.6.0"
 prance = "^0.21.8"