pip prod(deps): bump httpx from 0.20.0 to 0.21.3 (#951)

Bumps [httpx](https://github.com/encode/httpx) from 0.20.0 to 0.21.3.
- [Release notes](https://github.com/encode/httpx/releases)
- [Changelog](https://github.com/encode/httpx/blob/master/CHANGELOG.md)
- [Commits](encode/httpx@0.20.0...0.21.3)

---
updated-dependencies:
- dependency-name: httpx
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Abhinav Singh <126065+abhinavsingh@users.noreply.github.com>

Author: dependabot[bot]

requirements-testing.txt

@@ -16,7 +16,7 @@ mccabe==0.6.1
 pylint==2.12.2
 rope==0.22.0
 # Required by test_http2.py
-httpx==0.20.0
+httpx==0.21.3
 h2==4.1.0
 hpack==4.0.0
 hyperframe==6.0.1