Added skeletons for built-in string and bytes types
Added skeletons for built-in 'list', 'tuple' and 'dict' classes
Added skeletons for built-in 'file' class
Added mock class for <type 'generator'>
Separate 'builtins' skeleton for Python 3
Added skeletons for io.FileIO, io.TextIOWrapper and several others
Weaker type for round() built-in (PY-9072)