Fixes the following issues:
* Literals rather than ... for default values
* None rather than ... for default value of typed variable
* Literals rather than ... # type for top level constants
* # Foo rather than # type: Foo
* return value of init not set to None
This is the other half of fixing #1115.
Also fixed the stub for JSONDecodeError and made it only exist in 3.5+.
Didn't otherwise review the exact attributes and types in the package.