Add unary plus for operator function `(+)` when passing 1 argument

Python document says there is [unary plus operator](https://docs.python.org/3/reference/expressions.html#unary-arithmetic-and-bitwise-operations) in python. It's equivalent to `operator.pos`, and it's function can be overrided by implementing magic method `__pos__`.

While it's hard to find any real usage of unary plus operator, making `(+)` accept both 1 and 2 arguments is symmetric to `(-)`, which can be `operator.neg` with 1 argument or `operator.sub` with 2 arguments.