upgrade to using prompt_toolkit v2

[xyb]

New prompt_toolkit v2 released which not backward compatible. Upgrade needed.

see also #31 and 3f3cdb6

[stdedos]

I know this won't make much difference, but prompt-toolkit < 2 blocks from installing ipython fyi

[xyb]

@stdedos I have released a new version 2.0.0 which using prompt-toolkit 2.x