Fix test dependency: replace httpx-mock with pytest-httpx

The package httpx-mock doesn't exist. The correct package for mocking 
httpx in pytest is pytest-httpx which provides the httpx_mock fixture.

pyproject.toml

@@ -70,7 +70,7 @@ test = [
     "pytest>=7.0.0",
     "pytest-asyncio>=0.21.0",
     "pytest-cov>=4.0.0",
-    "httpx-mock>=0.10.0"
+    "pytest-httpx>=0.21.0"
 ]
 
 [project.urls]