Replace pkg_resources with importlib_metadata

This is using the new "selection interface" ( by calling `entry_points` with
the `group` parameter) that is being introduced in `importlib.metadata` of
Python 3.10 (currently in alpha). While the backport (`importlib_metadata`)
introduced the same change in version 3.6, various improvements have been made
up through the current release (3.10). Therefore we require
`importlib_metadata>=3.10` for all supported versions of Python (3.6-3.9).

requirements/project-min.txt

@@ -8,3 +8,4 @@ mdx_gh_links==0.2
 ghp-import==1.0
 pyyaml_env_tag==0.1
 mkdocs-redirects==1.0.1
+importlib_metadata==3.10.0

requirements/project.txt

@@ -8,3 +8,4 @@ mdx_gh_links>=0.2
 ghp-import>=1.0
 pyyaml_env_tag>=0.1
 mkdocs-redirects>=1.0.1
+importlib_metadata>=3.10