ENH: return an OrderedDict from read_excel with sheetname=None
 - [x] closes #9930
 - [x] tests added / passed
 - [ ] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry


## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14571?src=pr) is 85.27% (diff: 100%)

> Merging [#14571](https://codecov.io/gh/pandas-dev/pandas/pull/14571?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14571   diff @@
==========================================
  Files           140        140          
  Lines         50693      50693          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43229      43230     +1   
+ Misses         7464       7463     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [e1cdc4b...1a037a6](https://codecov.io/gh/pandas-dev/pandas/compare/e1cdc4b83d7e970e9683056be722d5fa2a00fa70...1a037a63d7b4720f78a4858f6434dada665a7441?src=pr)
 @scls19fr Thanks a lot!
