In python 3, add an overload for there being no digits argument
and make it return int.
In python 2, __round__ doesn't exist and SupportsRound doesn't exist
in the typing module. Use SupportsFloat for python 2 round().
Remove decimal's __round__ overload that takes None, since it doesn't exist
* Use Tuple field in DecimalTuple
* Remove unnecessary base classes from Decimal
* Decimal.__init__ -> __new__
* Decimal.__ne__ is not defined in Python 3
* Add Decimal.as_integer_ratio()
* Annotate DecimalException.handle()
* Correct types of Decimal method arguments
* Add missing arguments and optional markers to Decimal
* Add missing arguments to Context
* Remove spurious int from Unions with float
* Remove Context.__setattr__()
* Fix return types of Context methods
