[BUG] Missing dependecy HTTPX

[fedemarche]

Trying to do the following import:
from starlette.testclient import TestClient

will raise:
ModuleNotFoundError: No module named 'httpx'

starlette version: '0.21.0'

[alex-oleshkevich]

This is an optional dependency. Starlette does not include it by default because it is only needed it you want to run tests.