Add missing test dependency (httpx)

[EtiennePelletier]

Following the testing instructions to run the tests from a clean environment fail at the last step due to a missing httpx dependency.

To reproduce:

$ python3 -m venv .venv
$ . .venv/bin/activate
$ pip install -e .[tests]
$ pytest

ImportError while loading conftest 'jinja2-fragments/tests/conftest.py'.
tests/conftest.py:7: in <module>
    from fastapi.testclient import TestClient
.venv/lib/python3.10/site-packages/fastapi/testclient.py:1: in <module>
    from starlette.testclient import TestClient as TestClient  # noqa
.venv/lib/python3.10/site-packages/starlette/testclient.py:16: in <module>
    import httpx
E   ModuleNotFoundError: No module named 'httpx'

Considering that FastAPI's test client directly reuses Starlette's, it's best to add Starlette's test dependencies than FastAPI's (that has a very long list).

🐍 🤹‍♂️

[sponsfreixes]

Very good assessment. Thanks!