pyextract: do not use "is not" with a literal

[remytms]

I get this warning when running pyextract:

/home/remy/dev/marineelectronics/pyrenderer/pyextract.py:102: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if args.x is not -1 and args.y is not -1:
/home/remy/dev/marineelectronics/pyrenderer/pyextract.py:102: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if args.x is not -1 and args.y is not -1:

Do you accept pull request? I can be in charge of fixing it.