Replace httpx with httpx2

[stevepiercy]

When running the tests in CI, I noticed this warning message.

============================== warnings summary ===============================
.venv\Lib\site-packages\fastapi\testclient.py:1
  D:\a\icalendar-anonymizer\icalendar-anonymizer\.venv\Lib\site-packages\fastapi\testclient.py:1: StarletteDeprecationWarning: Using `httpx` with `starlette.testclient` is deprecated; install `httpx2` instead.
    from starlette.testclient import TestClient as TestClient  # noqa

httpx2 is the successor to httpx. See:

• https://tildeweb.nl/~michiel/httpx2.html
• https://github.com/pydantic/httpx2