Clarify `pkg_resources` vs `importlib_metadata`

[juftin]

pkg_resources is deprecated, it would be nice have the docs clarify how to use click-plugins with importlib_metadata / importlib.metadata instead.

ATTENTION

Use of pkg_resources is deprecated in favor of importlib.resources, importlib.metadata and their backports (importlib_resources, importlib_metadata). Some useful APIs are also provided by packaging (e.g. requirements and version parsing). Users should refrain from new usage of pkg_resources and should work to port to importlib-based solutions.

I've replaced this:

from pkg_resources import iter_entry_points

import click
from click_plugins import with_plugins


@with_plugins(iter_entry_points('core_package.cli_plugins'))
@click.group()
def cli():
    """Commandline interface for yourpackage."""

with this:

import click
from click_plugins import with_plugins
from importlib_metadata import entry_points


@with_plugins(entry_points(group='core_package.cli_plugins'))
@click.group()
def cli():
    """Commandline interface for yourpackage."""

The above code is compatible with importlib-metadata>=3.6

[geowurster]

I have switched to importlib in #40, but this PR comes with some major caveats.