Request for Adding `__version__` Attribute

[LakshmanKishore]

Summary:
Currently, the HDBSCAN library does not provide a __version__ attribute, which is a standard way for users to check the library's version. This issue is raised to request the addition of a __version__ attribute to HDBSCAN.

Details:
The __version__ attribute allows users to easily check the version of a library, which is a common and convenient practice in Python. For example, other libraries like joblib provide this attribute, making it simple to retrieve the version information. Adding a __version__ attribute to HDBSCAN would enhance the user experience and make it easier for users to verify the installed version.

Expected Behavior:
After this enhancement is implemented, users should be able to check the HDBSCAN version using the hdbscan.__version__ attribute.

[LakshmanKishore]

I have raised the pull request #615 to resolve this issue.