BUG: Allow non-callable attributes in aggregate function. Fixes GH16405
 - [x] closes #16405
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry

Can you add the test from the original issue here? It can go in `pandas/tests/frame/test_apply.py`

Also could use a release note in `doc/source/whatsnew/v0.20.2.txt`, under the bugfix section. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16458?src=pr&el=h1) Report
> Merging [#16458](https://codecov.io/gh/pandas-dev/pandas/pull/16458?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e81f3cc30725443a1d36ec27279f3fec4eed60ec?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16458/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/16458?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16458      +/-   ##
==========================================
+ Coverage   90.43%   90.43%   +<.01%     
==========================================
  Files         161      161              
  Lines       51045    51049       +4     
==========================================
+ Hits        46161    46165       +4     
  Misses       4884     4884
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.27% <100%> (ø)` | :arrow_up: |
| #single | `40.16% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16458?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/16458?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `96.21% <100%> (+0.03%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16458?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16458?src=pr&el=footer). Last update [e81f3cc...0a367c6](https://codecov.io/gh/pandas-dev/pandas/pull/16458?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I may have gone a bit overboard with the tests... although somehow I made coverage worse overall? @jreback are you OK with this? I believe the discussion was whether we could have a simpler implementation by special-casing `'size'`? can't change size as it's already defined as a property (for numpy compat)

 thanks!