Bump httpx from 0.15.0 to 0.15.3 (#155)

Bumps [httpx](https://github.com/encode/httpx) from 0.15.0 to 0.15.3.
- [Release notes](https://github.com/encode/httpx/releases)
- [Changelog](https://github.com/encode/httpx/blob/master/CHANGELOG.md)
- [Commits](encode/httpx@0.15.0...0.15.3)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

test-requirements.txt

@@ -19,7 +19,7 @@ filelock==3.0.12          # via pytest-mypy
 h11==0.9.0                # via httpcore, uvicorn
 httpcore==0.11.0          # via httpx
 httptools==0.1.1          # via uvicorn
-httpx==0.15.0             # via -r test-requirements.in
+httpx==0.15.3             # via -r test-requirements.in
 idna==2.10                # via requests, rfc3986, yarl
 iniconfig==1.0.1          # via pytest
 more-itertools==8.5.0     # via pytest