Update changelog.

CHANGES.txt

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 Chaco CHANGELOG
 ===============
 
-This is an incremental release which modernises the usage of Traits in a
-number of places, and fixes a number of bugs.
+Release 6.0.0
+-------------
+
+This is a major release which includes support for Python 3.11, and more recent
+versions of PySide6, TraitsUI and Enable.  There are no substantial new
+features, but some backwards compatibility support has been removed as
+advertised in deprecation warnings.  Substantial work has been done to ensure
+that demos and examples are working and up-to-date where possible.
+
+As usual, this release includes a number of bugfixes and minor improvements.
+
+Thanks to all who contributed to the release:
+
+* Chengyu Liu
+* Didrik Pinte
+* Corran Webster
+
+Enhancements
+
+* Remove deprecated stub files (#903)
+* Replace "modern" font with "san-serif" (#901)
+
+Fixes
+
+* Update textPlot when aesthetics change (#905)
+* Fix Wx Bitmap usage (#904)
+* Fix imports of `marker_trait` (#902)
+* Fire `updated` trait and Ranges when data sources are emptied (#900)
+* Fix LegendHighlighter hit detection (#897)
+* Properly handle NaN in BarPlots (#895)
+* Fix polar line renderer (#883)
+
+Documentation/Examples
+
+* Fix imports of ufuncs in Scalar Image Function Inspector example (#894)
+* Various small fixes for examples (#859, #864, #870, #871, #873, #874, #880, #882, #884, #885, #892, #893)
+
+Build/CI
+
+* Update testing infrastructure, including Python 3.11 (#891, #898)
+
+Release 5.1.1
+-------------
+
+This is a small bugfix release centered around Python 3.8+ support.
+
+Thanks to all who contributed to the release:
+
+* Aaron Ayres
+* Rahul Poruri
+* Corran Webster
+
+Fixes
+
+* Fix various warnings on Python 3.8 (#856)
+* Remove uses of deprecated np.bool (#852)
+* Prevent exceptions when plot size too small (#848)
+
+Build/CI
+
+* Get CI running on Python 3.8 (#851)
+* Fix CI for changes to GitHub infrastructure (#849)
+
 
 Release 5.1.0
 -------------
 
+This is an incremental release which modernises the usage of Traits in a
+number of places, and fixes a number of bugs.
+
 Thanks to all who contributed to the release:
 
 * Chris Angell