[clang-tidy] Enable Python 3 support for add_new_check.py

Summary: In Python 3, filters are lazily evaluated and strings are not bytes.

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: xazax.hun, cfe-commits

Differential Revision: https://reviews.llvm.org/D44217

llvm-svn: 328418