fix: make plugin module more robust and make yamllint works even if s…

…etuptools was not installed

setup.cfg

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

yamllint/plugins.py

@@ -17,7 +17,12 @@
 Plugin module utilizing setuptools (pkg_resources) to allow users to add their
 own custom lint rules.
 """
-import pkg_resources
+try:
+    from pkg_resources import iter_entry_points
+except ImportError:
+    def iter_entry_points(group):
+        """Dummy function for pkg_resources.iter_entry_points."""
+        yield
 
 
 PACKAGE_GROUP = "yamllint.plugins"
@@ -32,11 +37,11 @@ def validate_rule_module(rule_mod):
 
 def load_plugin_rules_itr(group=PACKAGE_GROUP):
     """Load custom lint rule plugins."""
-    for entry in pkg_resources.iter_entry_points(group):
+    for entry in iter_entry_points(group):
         try:
             rules_mod = entry.load()
             yield rules_mod.RULES_MAP
-        except ImportError:
+        except AttributeError:
             pass  # TBD