* make io classes inherit from typing IO classes
This makes these classes usable if type annotations are given as "IO"
or "TextIO". In the future, we'll then be able to move open() to
return a concrete class instead (#3951).
* open: introduce concrete return types
Fixes#3951.
We use the values of the "mode" and "buffering" arguments to figure out
the concrete type open() will return at runtime. (Compare the CPython
code in https://github.com/python/cpython/blob/master/Modules/_io/_iomodule.c#L231.)
* make io.pyi import _io.py(i), like io.py does
* make write/writelines take 'Any', on _IOBase
* Add missing constructors, fix inconsistencies.
* Also, as far as possible, try to simplify, by moving methods into base
classes.
* fix lint+mypy warnings
* add missing __enter__ methods
* make _IOBase inherit from BinaryIO
* make _TextIOBase not subclass _IOBase
Also fix signature of IO.seek, IO.truncate, IO.write, and MutableMapping.update
Fixes#1016
Note: I couldn't put typing.pyi in 2and3 because of an import cycle when adding `import sys` to 2/typing.pyi
