replace redis client for improved reliability

Added

• Parse pull request body to find dependabot versions. (#845)

Changed

• Replaced asyncio-redis with redis-py asyncio to improve network reliability. (#867)

Fixed

• Delete subscriptions from web API db on cancellation. (#838)
• Add workaround for bug in GitHub GraphQL API. (#842)
• Retry on merge being blocked to workaround GitHub API consistency issues. (#846)
• Fix schema to handle null requestedReviewer. (#847)
• Add better support for required codeowner reviews. (#849)
• Fix HTTP performance with httpx by reusing the ssl context. (#852)