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
