* 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
