BUG: Fix to GH34422 SeriesGroupBy works only with 'func' now
- [X] closes #34422 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] whatsnew entry

This PR tries to fix the bug pointed in #34422 where `SeriesGroupBy.agg` works with any given column name in `NamedAgg`.

After discussing with @TomAugspurger  and @MarcoGorelli  in another issue #34380 regarding this issue in the comments, We came to a solution that

> So for SeriesGroupBy.agg, if there are any tuples or NamedAgg present in kwargs then I think we should raise.
> Disallowing `NamedAgg` and tuples with `SeriesGroupBy`.

#### Before fix:
```python3
s = pd.Series([1,1,2,2,3,3,4,5])
s.groupby(s.values).agg(one = pd.NamedAgg(column='anything',aggfunc='sum'))
  one
1    2
2    4
3    6
4    4
5    5

s.groupby(s.values).agg(one=('something','sum'))
   one
1    2
2    4
3    6
4    4
5    5
```

#### After fix:
```python3
s = pd.Series([1,1,2,2,3,3,4,5])
s.groupby(s.values).agg(one = pd.NamedAgg(column='anything',aggfunc='sum'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "d:\#gh34422\pandas\pandas\core\groupby\generic.py", line 243, in aggregate
    raise TypeError(tuple_given_message.format(type(kwargs[col]).__name__))
TypeError: 'func' is expected but recieved NamedAgg in **kwargs.

s.groupby(s.values).agg(one=('something','sum'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "d:\#gh34422\pandas\pandas\core\groupby\generic.py", line 243, in aggregate
    raise TypeError(tuple_given_message.format(type(kwargs[col]).__name__))
TypeError: 'func' is expected but recieved tuple in **kwargs.
```
Hello @gurukiran07! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:







There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-06-03 19:37:24 UTC Thanks for the PR

Can you add a test as well to show how it would close the linked issue? @MarcoGorelli I'm new to open source development. I'm going through [writing-tests](https://github.com/pandas-dev/pandas/blob/master/doc/source/development/contributing.rst#writing-tests) mentioned in contributing.rst . I'll add tests by tomorrow.

It's currently showing failed in 9 checks and 2 successful checks. How and where can I fix those 9 failing checks? If you click through on the "Details" you should see the failures. e.g. https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=36338&view=logs&j=bef1c175-2c1b-51ae-044a-2437c76fc339&t=770e7bb1-09f5-5ebf-b63b-578d2906aac9&l=105 @jreback and @MarcoGorelli Added tests in `test_aggregate.py`. Our documentation has contributing guidelines. Check those at let me know if you still have issues. 

> On May 29, 2020, at 05:12, guru kiran <notifications@github.com> wrote:
> 
> ﻿
> @TomAugspurger Thank you. I still have 1 failing check in Linting. How can I fix it? Please excuse me for asking so many doubts. I'm 18 years old with no prior experience in open source development. Thank you.
> 
> —
> You are receiving this because you were mentioned.
> Reply to this email directly, view it on GitHub, or unsubscribe.
 @TomAugspurger Thank you. Now, all checks have been passed. Should have gone through "Coding style" part of contribution guidelines before. No, strings aren't callable. So it'd need to be a callable or str? Perhaps
we already have something to check that.

On Mon, Jun 1, 2020 at 3:25 PM guru kiran <notifications@github.com> wrote:

> *@gurukiran07* commented on this pull request.
> ------------------------------
>
> In pandas/core/groupby/generic.py
> <https://github.com/pandas-dev/pandas/pull/34435#discussion_r433469668>:
>
> >          if relabeling:
>              columns = list(kwargs)
> -            func = [kwargs[col] for col in columns]
> +            func = []
> +            for col in columns:
> +                if isinstance(kwargs[col], (list, tuple)):
>
> @TomAugspurger <https://github.com/TomAugspurger> aggfunc can be str
> sometimes right.
>
> s.groupby(...).agg(one='sum',two='max)
>
> Will callable return True the above example?
>
> —
> You are receiving this because you were mentioned.
> Reply to this email directly, view it on GitHub
> <https://github.com/pandas-dev/pandas/pull/34435#discussion_r433469668>,
> or unsubscribe
> <https://github.com/notifications/unsubscribe-auth/AAKAOITWLMYV7XWCPRMW6NLRUQFDNANCNFSM4NNKSIPQ>
> .
>
 @TomAugspurger 
> No, strings aren't callable. So it'd need to be a callable or str? Perhaps we already have something to check that.

`str` func is handled using `getattr` in the below lines may be we can use `hasattr(self,col_func)`?


 @TomAugspurger 

```python
if isinstance(func,str):
    return getattr(self,func)(*args,**kwargs)
```
These lines are hit when `str` is passed. I guess we can use `hasattr` or `callable`. @jreback all 14 checks passed. And updated whatsnew `v1.1.0.rst` under groupby bug fixes. @jreback All checks passed. Updated with mentioned changes. thanks @gurukiran07 very nice! @jreback Thank you. It was a huge learning experience, got to learn lot of new stuff. Hope to contribute more in future. 