Fix python 2-vs-3 issues in add_new_check.py and rename_check.py

As of this commit:
  https://github.com/llvm/llvm-project/commit/307b1fdd

If either of those scripts are invoked with python 2, neither works due to:
  "TypeError: write() argument 1 must be unicode, not str"

And if rename_check.py is invoked with python 3:
  "ValueError: binary mode doesn't take an encoding argument"
(referring to `with io.open(filename, 'wb', encoding='utf8') as f:`), and

Another issue in rename_check.py in python 2:
  "TypeError: list object is not an iterator"
(referring to `next(filter( ... os.listdir(old_module_path)))`)

(so, rename_check doesn't work with either 2 or 3, and add_new_check
doesn't work with 2, but does work with 3)

I ran these steps to test both python versions:
(manually - appears to be the "status quo" for these files)

  python3 clang-tools-extra/clang-tidy/add_new_check.py readability ggggg
  python3 clang-tools-extra/clang-tidy/rename_check.py readability-ggggg readability-hhhhh

  git checkou...