fix: add runtime dependency to setuptools

yamllint depends on pkg_resources.load_entry_point from setuptools to
make its command working, so this runtime dependency to setuptools is
necessary to be listed.

setup.cfg

@@ -54,6 +54,7 @@ include_package_data = True
 install_requires =
   pathspec >= 0.5.3
   pyyaml
+  setuptools
 
 test_suite = tests