Emmett 2.1 Bell

Changes since 2.0:

• Added type hints on all major interfaces
• Deprecated App.run
• Deprecated string signals in favour of extensions.Signals
• Removed libs.sanitizer
• Use orjson for JSON serialization
• Refactored request flow internals
• Added namespaces to templater
• Added SameSite support to session cookies
• Added HTTP/2 support
• Added Request.push_promise
• Added Python 3.9 support (beta)