py3: add pycompat.open and replace open() calls open() requires mode argument as unicodes on Python 3. This patch introduces pycompat.open() which is inserted to files using transformer and replaces builtins.open() calls.

localrepo: deprecate 'repo.opener' (API) The "new" 'repo.vfs' attribute have been around for almost 5 years. I think we can deprecate the old form now ;-)