[opt-viewer] Python 3 support in opt-stats.py

Summary: Minor changes that allow opt-stats.py to support both Python 2 and 3.

Reviewers: anemet, davidxl

Reviewed By: anemet

Subscribers: llvm-commits, fhahn

Differential Revision: https://reviews.llvm.org/D34564

llvm-svn: 306306