build(deps): update prompt-toolkit requirement from <3.0.0,>=2.0.0 to >=2.0.0,<4.0.0

[dependabot-preview]

Updates the requirements on prompt-toolkit to permit the latest version.

Changelog

Sourced from prompt-toolkit's changelog.

3.0.0: 2019-11-24

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.

2.0.9: 2019-02-19

Bug fixes:

• Fixed Application.run_system_command on Windows.
• Fixed bug in ANSI text formatting: correctly handle 256/true color sequences.
• Fixed bug in WordCompleter. Provide completions when there's a space before
  the cursor.

2.0.8: 2019-01-27

Bug fixes:

• Fixes the issue where changes made to the buffer in the accept handler were
  not reflected in the history.
• Fix in the application invalidate handler. This prevents a significat slow
  down in some applications after some time (especially if there is a refresh
  interval).
• Make print_container utility work if the input is not a pty.

New features:

• Underline non breaking spaces instead of rendering as '&'.
• Added mouse support for radio list.
• Support completion styles for READLINE_LIKE display method.
• Accept formatted text in the display text of completions.
• Added a FuzzyCompleter and FuzzyWordCompleter.
• Improved error handling in Application (avoid displaying a meaningless

... (truncated)

Commits

• 4cbbf8b Fixed reStructuredText from README.rst.
• 90a9bf0 Added comments regarding Python versions.
• c8f5b5c Remove redundant license parameter
• 9d7e5c8 New key bindings for shift-based text selection in Emacs mode
• ebe2da0 A few bug fixes for patch_stdout.
• 13ed459 use the right global value for current_path.
• 8fbac04 Can't join MouseEventType with strings
• 3dd5923 Fixed grammar in "Input/output is not a terminal" error message.
• 1e4b44e Advanced tempfile
• d3ad16d Typing for stop_time
• Additional commits viewable in compare view

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[coveralls]

Coverage remained the same at 94.709% when pulling b32285b on dependabot/pip/prompt-toolkit-gte-2.0.0-and-lt-4.0.0 into 2886ad4 on master.

[dependabot-preview]

Looks like prompt-toolkit is no longer a dependency, so this is no longer needed.