REF/TYP: define NDFrame numeric methods non-dynamically
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

cc @simonjayhawkins along with #36989 this gets most of the generic.py mypy complaints (notwithstanding complaints about `cls.foo = foo` that can be addressed by changing to `setattr(cls, "foo", foo)`.

it'd be nice to have a less-verbose way of pinning the docstrings.  i haven't had any luck so far at making that work.