fix: backend/requirements.txt to reduce vulnerabilities (#785)

The following vulnerabilities are fixed by pinning transitive dependencies: - https://snyk.io/vuln/SNYK-PYTHON-AIOHTTP-15873704 - https://snyk.io/vuln/SNYK-PYTHON-AIOHTTP-15873731 - https://snyk.io/vuln/SNYK-PYTHON-AIOHTTP-15873732 - https://snyk.io/vuln/SNYK-PYTHON-AIOHTTP-15873733 - https://snyk.io/vuln/SNYK-PYTHON-AIOHTTP-15873734 - https://snyk.io/vuln/SNYK-PYTHON-AIOHTTP-15873735 - https://snyk.io/vuln/SNYK-PYTHON-AIOHTTP-15873736 - https://snyk.io/vuln/SNYK-PYTHON-AIOHTTP-15873737 - https://snyk.io/vuln/SNYK-PYTHON-AIOHTTP-15873738 - https://snyk.io/vuln/SNYK-PYTHON-AIOHTTP-15873739 Co-authored-by: snyk-bot <snyk-bot@snyk.io>