use getenv instead of accessing environ obj

[tomcsojn]

I had problems importing the package after manipulating the environ object, so the "default" keyword argument was not available anymore. In this case getenv will still work.
The module using getenv in all other occurences, so it also good to keep it consistent.

[jhamon]

Looking at the official Python docs for os.getenv, it says:

Note that since getenv() uses os.environ, the mapping of getenv() is similarly also captured on import, and the function may not reflect future environment changes.

So I'm not confident this change will resolve the problem you experienced. But I also think it's fine to make this change, to improve consistency of the code overall.