Linkchecker requires older Python version

> This program requires Python requests 2.2.0 or later.

The the way this message is written makes it easy to misread, but the package referred to is actually not python, but python-requests.

$ apt show python-requests | grep Description
Description: elegant and simple HTTP library for Python2, built for human beings

However,

$ apt policy python-requests
python-requests:
Installed: 2.18.1-1+ubuntu16.04.1+certbot+1
Candidate: 2.18.1-1+ubuntu16.04.1+certbot+1

so you do have a version of python requests that is later than 2.2.0. This is a bug[1] affecting the original linkchecker, which has apparently been abandonded[2] by the original maintainer. The project has been continued with a community fork, which you can install with

$ sudo apt remove linkchecker # if you still have the old version installed
$ sudo apt install git gcc python-requests python-argcomplete python-geoip clamav python-gtk2-dev
$ git clone https://github.com/linkchecker/linkchecker.git
$ cd linkchecker
$ python setup.py build
$ sudo python setup.py install

[1] https://github.com/wummel/linkchecker/issues/649
[2] https://github.com/wummel/linkchecker/issues/708
[3] https://github.com/linkchecker/linkchecker