[opt viewer] Python compat - decode/encode string

Summary:
Use io.open instead of codecs.open according to here
https://stackoverflow.com/questions/10971033/backporting-python-3-openencoding-utf-8-to-python-2

Add `u` prefix to string literal to make them utf-8 in python2.

Reviewers: anemet, serge-sans-paille

Reviewed by: serge-sans-paille

Differential Revision: https://reviews.llvm.org/D73011