CLN: Define and pin GroupBy properties without exec
- [x] closes #16959
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

In an unrelated branch I found that this exec-generated code is a PITA to debug.  So this PR finally gets rid of it.
thanks @jbrockmendel 