Bump httpx from 0.6.8 to 0.7.1

Bumps [httpx](https://github.com/encode/httpx) from 0.6.8 to 0.7.1.
- [Release notes](https://github.com/encode/httpx/releases)
- [Changelog](https://github.com/encode/httpx/blob/master/CHANGELOG.md)
- [Commits](encode/httpx@0.6.8...0.7.1)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

test-requirements.txt

@@ -20,8 +20,9 @@ flake8==3.7.8             # via pytest-flake8
 h11==0.8.1                # via httpx, uvicorn
 h2==3.1.1                 # via httpx
 hpack==3.0.0              # via h2
+hstspreload==2019.8.16    # via httpx
 httptools==0.0.13         # via uvicorn
-httpx==0.6.8
+httpx==0.7.1
 hyperframe==5.2.0         # via h2
 idna==2.8                 # via httpx, requests, yarl
 importlib-metadata==0.19  # via pluggy, pytest