py-prompt_toolkit2: updated to 3.0.0

3.0.0:
New features:
- (almost) 100% type annotated.
- Native asyncio instead of custom event loops.
- Added shift-based text selection (use shift+arrows to start selecting text).

Breaking changes:
- Python 2 support has been dropped. Minimal Python version is now 3.6,
  although 3.7 is preferred (because of ContextVars).
- Native asyncio, so some async code becomes slightly different.
- The active `Application` became a contextvar. Which means that it should be
  propagated correctly to the code that requires it. However, random other
  threads or coroutines won't be able to know what the current application is.
- The dialog shortcuts API changed. All dialog functions now return an
  `Application`. You still have to call either `run()` or `run_async` on the
  `Application` object.
- The way inputhooks work is changed.
- `patch_stdout` now requires an `Application` as input.

(adam)