Bump httpx in /{{cookiecutter.project_slug}}/backend (#132)

Bumps [httpx](https://github.com/encode/httpx) from 0.14.2 to 0.15.5.
- [Release notes](https://github.com/encode/httpx/releases)
- [Changelog](https://github.com/encode/httpx/blob/master/CHANGELOG.md)
- [Commits](encode/httpx@0.14.2...0.15.5)

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Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

{{cookiecutter.project_slug}}/backend/requirements.txt

@@ -3,7 +3,7 @@ Authlib==0.14.3
 fastapi==0.61.1
 celery==5.0.0
 redis==3.5.3
-httpx==0.14.2
+httpx==0.15.5
 ipython==7.18.1
 itsdangerous==1.1.0
 Jinja2==2.11.2