Update httpx requirement from >=0.15.4,<0.26.0 to >=0.15.4,<0.28.0 (#207)

Updates the requirements on [httpx](https://github.com/encode/httpx) to permit the latest version.
- [Release notes](https://github.com/encode/httpx/releases)
- [Changelog](https://github.com/encode/httpx/blob/master/CHANGELOG.md)
- [Commits](https://github.com/encode/httpx/compare/0.15.4...0.27.0)

---
updated-dependencies:
- dependency-name: httpx
  dependency-type: direct:production
...

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pyproject.toml

@@ -17,7 +17,7 @@ url = "https://pypi.org/simple/"
 
 [tool.poetry.dependencies]
 attrs = ">=20.1.0,<24.0.0"
-httpx = ">=0.15.4,<0.26.0"
+httpx = ">=0.15.4,<0.28.0"
 python = "^3.8"
 python-dateutil = "^2.8.0"
 websockets = "^11.0.3"