Fix a Python 3 warning
Fix a Python 3 warning

```
/home/simon/projects/servo/python/servo/testing_commands.py:301:
SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if err is not 0:
```

DominoTree/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,DominoTree/servo,DominoTree/servo,KiChjang/servo,splav/servo,splav/servo,DominoTree/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,DominoTree/servo,KiChjang/servo,splav/servo,splav/servo,splav/servo,KiChjang/servo,splav/servo,splav/servo,DominoTree/servo,DominoTree/servo,splav/servo,DominoTree/servo,splav/servo,splav/servo,DominoTree/servo,DominoTree/servo,KiChjang/servo