Update httpx requirement from <0.28 to <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@0.0.1...0.28.0)

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

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

setup.py

@@ -76,7 +76,7 @@
               'typesystem<0.3',
           ],
           'test': [
-              'httpx<0.28',
+              'httpx<0.29',
               'pytest<9',
               'pytest-cov<7',
           ],