When using Python 3, pygments.highlight() returns a bytes object, not
a str, causing the call to str.replace on the following line to fail
with a runtime exception:
TypeError: 'str' does not support the buffer interface. Decode the
bytes into a string in order to fix the exception.
Test Plan:
Run opt-viewer.py with Python 3.4, and confirm no runtime error occurs
when calling str.replace.