Groupby transform preserves output dtype
Previously `transform` output was always the same dtype as the groupby
object. This allows the output dtype to differ from the input. Fixes #9807.

looks great! could you add a release note to what's new?
 merged via https://github.com/pydata/pandas/commit/30580e7acd6cafd55616d91bedbb8999d65177f5
thanks!
