What is "pkg-resources==0.0.0" in output of pip freeze command

As for the part of your question "is it OK to remove this line?":

I have the same issue here developing on an ubuntu 16.04 with that very line in the requirements. When deploying on a debian 8.5 running "pip install -r requirements.txt" pip complains that pkg-resources is "not found" but there is a global package installed "python-pkg-resources" so the dependency should be satisfied. Same on ubuntu: The package exists there as well.

As stated here it seems to be some "implicitly installed package".

So: If you are on a Debian/Ubuntu having python-pkg-resources installed it should be safe to remove that line. I did so and everything is running fine. However since I am no expert on this you should keep in mind that this might lead to complications when deploying on another machine.

According to https://github.com/pypa/pip/issues/4022, this is a bug resulting from Ubuntu providing incorrect metadata to pip. So, no there does not seem to be a good reason for this behaviour. I filed a follow-up bug with Ubuntu. https://bugs.launchpad.net/ubuntu/+source/python-pip/+bug/1635463

To backup the previous answer, it should be safe to remove that line from your requirements.txt. Here is an example Make file stanza that safely freezes your package list (drop in your Makefile and run with make freeze):

freeze:
    pip freeze | grep -v "pkg-resources" > requirements.txt

edit 2022 July 06:

I have been informed that the package name differs depending on the system in use (pkg-resources vs pkg_resources). Please see the comments attached to this answer for differences in usage between different versions of Debian/Ubuntu. As pkg-resources is the historically correct package name at the time this was posted (almost 6 years ago) for the system in question, it will remain unchanged in this answer.