Update httpx requirement from >=0.23.1,<0.28.0 to >=0.23.1,<0.29.0

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](encode/httpx@0.23.1...0.28.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

pyproject.toml

@@ -22,7 +22,7 @@ frozendict = ">=2.0.2,<3"
 starlette = ">=0.27.0,<1"
 
 [tool.poetry.group.dev.dependencies]
-httpx = ">=0.23.1,<0.28.0"
+httpx = ">=0.23.1,<0.29.0"
 mkdocs-material = ">=8.3.9,<10.0.0"
 pytest = ">=7,<9"
 pytest-cov = ">=3,<6"