Emmett 2.2 Copernicus

Changes since 2.1:

• Slightly refactored request flow
• Added App.command_group decorator
• Added additional arguments acceptance in AppModule
• Added static paths customisation for AppModule
• Added workers options to serve command
• Changed default logging level to info in serve command
• Changed default SameSite policy for session cookies to Lax
• Added official Python 3.9 support