Resolve "Single-source the package version"

What does this MR do?

This MR implements the find_version function, which is used in setup.py and doc/conf.py to extract the version string from dantro/__init__.py, rather than specifying it in three places at once.

Can this MR be accepted?

• Implemented the changes
• Tested locally and in pipeline
• Pipeline passes
• Changelog entry added

Anything to double-check?

Nope.

Related issues

Closes #44 (closed)