Since we have a ticket to track dropping Python 2 support in#819 in 0.21 I thought it'd be OK to have a ticket to track dropping support for the various backends bpython currently supports.

The current backends we 'support' (they are in various state of support hence the scare quotes):

1. curtsies
2. cli (deprecation warning since 0.19)
3. urwid (deprecation warning since 0.19)

Of these I believe the curses backend is required to support Windows, the urwid backend is required to support integration with Twisted's reactor (see#803).

I'd want to support both of these usecases in the curtsies frontend, I personally mostly care about integration with event loops as Python has added this natively by running -m asyncio.

Is there anything I am lacking in regards to unique features so I can make a plan of attack@sebastinas@thomasballinger ?

More related tickets:#797