BUG: unicode characters when reading JSON lines
Fixes UnicodeDecodeError bug when reading JSON lines input with Ascii decoder, which is often the default setting in Python 2.7. Avoids issues with mixing unicode and ascii strings.

xref #15132

 - [x] closes #15132
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [x] whatsnew entry

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/15149?src=pr) is 85.54% (diff: 100%)
> Merging [#15149](https://codecov.io/gh/pandas-dev/pandas/pull/15149?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #15149   diff @@
==========================================
  Files           145        145          
  Lines         51404      51404          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43975      43976     +1   
+ Misses         7429       7428     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [8e13da2...e117889](https://codecov.io/gh/pandas-dev/pandas/compare/8e13da24818dffaec88321a75a1e728b50a642f2...e1178894a9f74a3e2e3369f5cd36624205085b36?src=pr) lgtm. ping on green. @jreback: all green here. thanks!