Ptpython

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Fixes:
- Further improve performance of dictionary completions.

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New features:
- Custom 'exit' function to return from REPL that
* doesn't terminate `sys.stdin` when `exit` is called (important for
`embed()`).
* doesn't require to be called with parentheses.

Fixes:
- Clean up signatures on control-c.

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- Limit number of completions to 5k (for performance).
- Several typing fixes.

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Fixes:
- Handle `GeneratorExit` exception when leaving the paginator.

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Fixes:
- Fix handling of 'config file does not exist' when embedding ptpython.

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Fixes:
- Don't show "Impossible to read config file" warnings when no config file was
passed to `run_config()`.
- IPython integration fixes:
* Fix top-level await in IPython.
* Fix IPython `DeprecationWarning`.
- Output printing fixes:
* Paginate exceptions if pagination is enabled.
* Handle big outputs without running out of memory.
- Asyncio REPL improvements:
* From now on, passing `--asyncio` is required to activate the asyncio-REPL.
This will ensure that an event loop is created at the start in which we can
run top-level await statements.
* Use `get_running_loop()` instead of `get_event_loop()`.
* Better handling of `SystemExit` and control-c in the async REPL.