Bump httpx from 0.22.0 to 0.23.0 in /tests

Bumps [httpx](https://github.com/encode/httpx) from 0.22.0 to 0.23.0.
- [Release notes](https://github.com/encode/httpx/releases)
- [Changelog](https://github.com/encode/httpx/blob/master/CHANGELOG.md)
- [Commits](encode/httpx@0.22.0...0.23.0)

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

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

Author: dependabot[bot]

tests/requirements.txt

@@ -1,5 +1,5 @@
 tox==3.24.3
 trio>=0.19.0
 pytest==6.2.5
-httpx==0.22.0
+httpx==0.23.0
 appengine-python-standard  # for loading the gae backend