Prompt-toolkit

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Fixes:
- Make the flush method of the vt100 output implementation re-entrant (fixes an
issue when using aiogevent).
- Fix off-by-one in `FormattedTextControl` mouse logic.
- Run `print_container` always in a thread (avoid interfering with possible
event loop).
- Make sphinx autodoc generation platform agnostic (don't import Windows stuff
when generating Sphinx docs).

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New features:
- Added `in_thread` parameter to `Application.run`.
This is useful for running an application in a background thread, while the
main thread blocks. This way, we are sure not to interfere with an event loop
in the current thread. (This simplifies some code in ptpython and fixes an
issue regarding leaking file descriptors due to not closing the event loop
that was created in this background thread.)

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New features:
- Accept `style` parameter in `print_container` utility.
- On Windows, handle Control-Delete.

Fixes:
- Avoid leaking file descriptors in SSH server.

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New features:
- Added `ScrollablePane`: a scrollable layout container.
This allows applications to build a layout, larger than the terminal, with a
vertical scroll bar. The vertical scrolling will be done automatically when
certain widgets receive the focus.
- Added `DeduplicateCompleter and `ConditionalCompleter`.
- Added `deduplicate` argument to `merge_completers`.

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Fixes:
- Set stdout blocking when writing in vt100 output. Fixes an issue when uvloop
is used and big amounts of text are written.
- Guarantee height of at least 1 for both labels and text areas.
- In the `Window` rendering, take `dont_extend_width`/`dont_extend_height` into
account. This fixes issues where one window is enlarged unexpectedly because
it's bundled with another window in a `HSplit`/`VSplit`, but with different
width/height.
- Don't handle `SIGWINCH` in progress bar anymore. (The UI runs in another
thread, and we have terminal size polling now).
- Fix several thread safety issues and a race condition in the progress bar.
- Fix thread safety issues in `Application.invalidate()`. (Fixes a
`RuntimeError` in some situations when using progress bars.)
- Fix handling of mouse events on Windows if we have a Windows 10 console with
ANSI support.
- Disable `QUICK_EDIT_MODE` on Windows 10 when mouse support is requested.

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New features:
- Disable bell when `PROMPT_TOOLKIT_BELL=false` environment variable has been
set.

Fixes:
- Improve cancellation of history loading.