Adjust testsuite to work with latest httpx

[trepel]

The httpx version has been pinned to 0.27.2 (#611) because of deprecation of MutableMaping in "extensions" and changes to ssl verify option in followup versions. Adjust the test suite so that the latest version can be used again.

https://github.com/encode/httpx/releases/tag/0.28.0

Potential ways to refactor:
https://github.com/encode/httpx/blob/0.28.0/docs/advanced/ssl.md