Change whitespace in subset.py to conform with Python style guide
Change whitespace in subset.py to conform with Python style guide

The Python style (outside of Google) is four spaces for indentation.
After this change, pylint still has complaints about this file,
but these formatting changes will probably improve the health
metrics of landscape.io. Personally, I couldn't care less about
whitespace, but it is hard to spot actual problems in a sea
of lint warnings.

fonttools/fonttools,googlefonts/fonttools