Replace use of pkg_resources with importlib.resources

[dvzrv]

Requested Feature: Since this project only officially supports Python >= 3.7, it is now possible to remove the setuptools dependency with the cpython builtin importlib.resources.

Related Area: dependencies

Do you want to contribute this yourself as a pull request? (don’t worry about it if you don’t want to/can’t — someone else can take care of it)

• Yes, I have already written code for it (link if available and feasible)
• Yes, I don’t have code ready yet (that’s okay!)
• No (that’s okay too!)

Does this feature affect backwards compatibility? If yes, in what way?

As this was introduced in Python 3.7, it should not have any side-effect!

Rationale and full description: (why should it be added to Nikola?)

The pkg_resources API is deprecated and replacing it with a Python internal library is a good approach :)