Update httpx requirement from <0.28,>=0.18 to >=0.18,<0.29

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@0.18.0...0.28.1)

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

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

pyproject.toml

@@ -81,7 +81,7 @@ classifiers = [
 ]
 requires-python = ">=3.9"
 dependencies = [
-    "httpx >=0.18,<0.28", # Starting 0.28, there is an incompatibility with respx: https://github.com/lundberg/respx/issues/277
+    "httpx >=0.18,<0.29", # Starting 0.28, there is an incompatibility with respx: https://github.com/lundberg/respx/issues/277
 ]
 
 [project.urls]