[opt-viewer] Decode HTML bytes for Python 3

Summary:
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`.

Reviewers: anemet

Reviewed By: anemet

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36624

llvm-svn: 310741