Add CHANGELOG entry for #647

Author: duckontheweb

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 - Added Table-extension ([#646](https://github.com/stac-utils/pystac/pull/646))
 - Stable support for Python 3.10 ([#656](https://github.com/stac-utils/pystac/pull/656))
+- `.python-version` files are now ignored by Git ([#647](https://github.com/stac-utils/pystac/pull/647))
 
 ### Removed