Update changelog for v3.9.0 release
## HEAD (Unreleased)
+## 3.9.0 (November 5, 2021)
+
- [sdk/python] Add ready attribute to await Helm charts (https://github.com/pulumi/pulumi-kubernetes/pull/1782)
- [sdk/go] Add ready attribute to await Helm charts (https://github.com/pulumi/pulumi-kubernetes/pull/1784)
- [sdk/dotnet] Add ready attribute to await Helm charts (https://github.com/pulumi/pulumi-kubernetes/pull/1785)
