Handle side-effects from writing to files

[khatchad]

We are detecting side-effects for calls to writelines(), though I don't think for the correct reasons, but write() isn't being detected as side-effect producing.

References

• https://docs.python.org/3/library/io.html#io.RawIOBase.write
• https://docs.python.org/3/library/io.html#io.IOBase.writelines