Prompt toolkit uses a very slow method to get the `__version__`

[joouha]

I've been doing some profiling of the start-up time of my prompt_toolkit application, and I noticed that prompt_toolkit spends a significant chunk of it's import time using importlib.metadata to retrieve the version number from the package distribution:

This seems unnecessary when the version number could simply be hard-coded in __init__.py, which would significantly reduce the time it takes to import prompt_toolkit.

Many modern packaging tools allow the version number to be dynamically sourced from __init__.py, rather than the other way round.

What are your thoughts about changing to a hard-coded version number?

[jonathanslenders]

That is only a small fraction of the whole package import.

I'd prefer it to not be hard-coded, but a module level __getattr__ would be fine for me. Feel free to create a PR.