BUG: allow describe() for DataFrames with only boolean columns
- [x] closes #13891
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Closes #13891. Existing tests pass. ~~No new tests added yet.~~

Thx for the PR! We appreciate including the tests from first to see the detail.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13898?src=pr) is 85.30% (diff: 100%)

> Merging [#13898](https://codecov.io/gh/pydata/pandas/pull/13898?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13898   diff @@
==========================================
  Files           139        139          
  Lines         50143      50143          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          42777      42776     -1   
- Misses         7366       7367     +1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [7e15923...26201aa](https://codecov.io/gh/pydata/pandas/compare/7e15923d908a42008fb7fecde58be626277fd82e...26201aa0a9e4d7f48410bf8f244ead9d682d200a?src=pr)
 @sinhrks Changes made.
 lgtm. ping on green.
 > ping on green.

?
 when all of the checks have passed it will turn grin. ping then.
 @jreback
 thanks @agraboso 
