Update httpx requirement from <1,>=0.27.0 to >=0.28.1,<1

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@0.27.0...0.28.1)

---
updated-dependencies:
- dependency-name: httpx
  dependency-version: 0.28.1
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements/base.txt

@@ -11,7 +11,7 @@ cryptography==47.0.0  # https://github.com/pyca/cryptography
 pydantic==2.*
 typing-extensions==4.*  # https://github.com/python/typing_extensions
 requests>=2.33.1,<3  # https://requests.readthedocs.io/en/latest/
-httpx>=0.27.0,<1  # https://github.com/encode/httpx - async HTTP for agent tools
+httpx>=0.28.1,<1  # https://github.com/encode/httpx - async HTTP for agent tools
 tokenizers>=0.21,<0.23  # Pin to prevent conflicts with transformers