fixes #534

pathlib._NormalAccessor wraps `open` in `staticmethod` for py < 3.7 but
not 3.7. That means we `Path.open` calls `green.os.open` with `file`
being a pathlib._NormalAccessor object, and the other arguments shifted.
Fortunately pathlib doesn't use the `dir_fd` argument, so we have space
in the parameter list. We use some heuristics to detect this and adjust
the parameters (without importing pathlib)