BUG: Fix for .extractall (single group with quantifier) #13382
- [X] closes #13382
- [X] tests added / passed
- [X] passes `git diff upstream/master | flake8 --diff`
- [X] whatsnew entry

Note that I had to fix it based on this [thread](http://stackoverflow.com/questions/2111759/whats-the-best-practice-for-handling-single-value-tuples-in-python), rather than directly with `[x for x in ['ab']]` as this broke previous tests. 

minor changes, looks good. ping when green.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13397?src=pr) is **84.23%**

> Merging [#13397](https://codecov.io/gh/pydata/pandas/pull/13397?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13397   diff @@
==========================================
  Files           138        138          
  Lines         50803      50805     +2   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42793      42795     +2   
  Misses         8010       8010          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [d5bea25...2afae62](https://codecov.io/gh/pydata/pandas/compare/d5bea25274b5faad5d2014b279a1da6451ebe1c7...2afae6272783f0fd3de76ae520b6bdf1e593eeca)
 @jreback Done. 
 thanks!
