Make run-clang-tidy compatible with Python 3.x

Reviewers: alexfh

Reviewed By: alexfh

Subscribers: cfe-commits, JDevlieghere

Tags: #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D37138

Change-Id: I89a95d1e082e566e7e64c2a5ca4123c543e6b1be
llvm-svn: 312532