Don't always force one-element-per-line when a sequence literal contains a comma after the last element

As of 0.21.0, yapf formats
```
_MY_CONSTANT = (3,4,5,6,7)
```
as
```
_MY_CONSTANT = (3, 4, 5, 6, 7)
```
and formats
```
_MY_CONSTANT = (3,4,5,6,7,)
```
as
```
_MY_CONSTANT = (
3,
4,
5,
6,
7,
)
```
(the only difference between the two inputs is the comma after the last element). I'd much rather the latter form, which I prefer writing, were laid out much more compactly. If there's any reason why it shouldn't be compact, I'm having trouble seeing it.

(We use yapf for all the Python code in [gRPC Python](https://github.com/grpc/grpc/tree/master/src/python) and this is my biggest complaint, so congratulations on everything else about it being great!)