[PERF] fixing memory leak in aggregation.pyx
- [x] closes #43339
- [x] tests passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew entry

The old memory needs to be released, before the pointer switches to the new list.

would be ok including this in 1.3.x if you can move the note cc @simonjayhawkins ok to include this for 1.3.4

 > cc @simonjayhawkins ok to include this for 1.3.4

sure. thanks @realead  @meeseeksdev backport 1.3.x Something went wrong ... Please have  a look at my logs.