bpo-43428: Improve documentation for importlib.metadata changes. (GH-24858)

* bpo-43428: Sync with importlib_metadata 3.7.3 (16ac3a95)

* Add 'versionadded' for importlib.metadata.packages_distributions

* Add section in what's new for Python 3.10 highlighting most salient changes and relevant backport.
3 files changed