Update httpx requirement from ~=0.25.0 to ~=0.25.1 (#501)

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@0.25.0...0.25.1)

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

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requirements_testing.txt

@@ -1,4 +1,4 @@
-httpx~=0.25.0
+httpx~=0.25.1
 pgtest~=1.3
 pytest~=7.4
 pytest-cov~=4.1