Python: Add change note

Author: tausbn

python/change-notes/2021-04-13-werkzeug-api-graphs.md

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+lgtm,codescanning
+* The Werkzeug model has been changed to use API graphs. When defining new models for classes based
+  on the `MultiDict` and `FileStorage` classes in `werkzeug.datastructures`, the relevant extension
+  points are now the two `InstanceSourceApiNode` classes in the `semmle.python.frameworks.Werkzeug`
+  module, instead of `InstanceSource`. The latter classes have now been deprecated.