Declare package version number on __init__

[andfoy]

Thanks for your great work on this library, at Spyder, we're currently using intervaltree to handle code folding region nesting spyder-ide/spyder#10333.

We use a package version checker at startup to see if users have a correct installed version of the required runtime libraries via <package name>.__version__, however when I look at the __init__.py file of this package, there is no reference to the current version of the package. The use of the __version__ attribute is customary across multiple third-party Python packages and libraries, and its use is recommended when redistributing a package: https://packaging.python.org/guides/single-sourcing-package-version/

I can help you with this addition, if you want.