Fix decimal (#2323)

* 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

Author: srittau