Add missing encoding=utf-8 to various open calls. This makes
files like spec.json, spack.yaml, spack.lock, config.yaml etc locale
independent w.r.t. text encoding. In practice this is not often an
issue since Python 3.7, where the C locale is promoted to
C.UTF-8. But it's better to enforce UTF-8 explicitly, since there is
no guarantee text files are written in the right encoding.
Also avoid opening in text mode if it can be avoided.
526af1cbe7