managing dev/null file add and removal

[memsharded]

Hi!

I am trying to contribute the behavior for adding/removing files, i.e. managing /dev/null.

Most likely this is a poor solution, but please let me know if some other approach is better:

• The test suite is a bit custom, I was having duplicated tests, I finally added an "example" folder, but should be easily moved to a test
• The system still warns about /dev/null abspath, but I wasn't confident enough to modify that part

Btw: thanks for sharing this useful library!

[memsharded]

It seems some CI is failing, with Py3, but not sure related to this PR: https://travis-ci.org/techtonik/python-patch/jobs/286056242

Could you please check? Thank you!

[uilianries]

Hi @techtonik ! Could you please review this PR?