Update package version to 2.9.1. (#263)

I think this will let source builds appear as newer than the stable PyPI
release that was just published
(https://pypi.org/project/iree-turbine/2.9.0/). We can iterate on the
release process sequencing next week. Maybe we shouldn't touch this
version, or maybe we should update the minor version instead of the
patch version, I'm not sure.

version_info.json

@@ -1,3 +1,3 @@
 {
-    "package-version": "2.9.0"
+    "package-version": "2.9.1"
 }