REGR: Allow positional arguments in DataFrame.agg
Although `DataFrame.agg` is documented as accepting `func, axis, *args, **kwargs` with `*args` and `**kwargs` passed to `func`, passing positional arguments raises a TypeError.

The reason for this is that the internal call to `self._aggregate` uses a keyword argument (axis) before passing *args and `**kwargs`, and as such the first positional argument is always interpreted as a second specification of `axis`, which raises TypeError.

Prior to commit 433c9007781080658553fbef1a4d0c2813b404c0, TypeErrors were being suppressed, falling back to `self.apply`, which in v1.1.0 turned into an error.

This fixes issue GH-36948.

- [x] closes #36948
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
