Update httpx requirement from <=0.27.2 to <=0.28.1

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@0.0.1...0.28.1)

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

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

requirements.txt

@@ -15,7 +15,7 @@ frozendict<=2.4.4
 h11<=0.14.0
 html5lib<=1.1
 httpcore<=1.0.6
-httpx<=0.27.2
+httpx<=0.28.1
 humanfriendly<=10.0
 idna<=3.10
 imageio<=2.35.1