CI: Use pylint, instead of pylint3

It seems, despite the name, both packages depend on Python 3. However,
pylint3 seems deprecated, and upstream recommends to install pylint:
https://www.pylint.org/#install

The current versions of both packages in Debian unstable are:
pylint  2.4.4-1
pylint3 2.2.2-1

This commit fixes failing CI jobs due to the use of pylint3 2.2.2-1,
which seems broken.