avoiding unwanted tuples with trailing comma

I just encountered a bug due to a trailing comma: i.e, in my coda I had 
```
FOO = 42,
```
instead of 
```
FOO = 42
```
It interpreted `FOO` as a tuple. It would be great to have a rule to avoid such bugs in future (maybe removing trailing commas? ) 

@CamDavidsonPilon 