Trailing commas are enforced in Python 2 after unpacking

[SalmonMode]

In Python 2, the following two function calls are invalid syntax:

some_function(
    *my_arguments,
)

some_function(
    *my_arguments,
    my_keyword_argument="some value",
)

But in Python 3, they are perfectly fine and the trailing comma should be enforced.

This plugin should detect what version of Python is being used, and should know whether or not to enforce the trailing comma in these circumstances depending on what version of Python it is.