Adding Ninja to module requirements in setup.up

[Hacker1337]

I see that there is pull request adding Ninja to requirements.txt file #66 but it seems like it is also reasonable to keep in in the requirements for python package (in setup.py file). I'm not sure about other packages mentioned in requirements.txt. May be they should be also added to setup.py if they are needed for package, but I had problems only with Ninja.