Revert "completely migrate from setuptools to importlib"

This reverts commit 79e53eb.

Author: GrahamDumpleton

src/wrapt/importer.py

@@ -93,19 +93,24 @@ def import_hook(module):
                 callback = getattr(callback, attr)
         return callback(module)
 
+    def import_hook_legacy(module):
+        __import__(entrypoint.module_name)
+        callback = sys.modules[entrypoint.module_name]
+        for attr in entrypoint.attrs:
+            callback = getattr(callback, attr)
+        return callback(module)
+    
+    if sys.version_info < (3, 10):
+        return import_hook_legacy
     return import_hook
 
 def discover_post_import_hooks(group):
-    from importlib.metadata import entry_points
-
-    try:
-        # Python 3.10+ style with select parameter
-        entrypoints = entry_points(group=group)
-    except TypeError:
-        # Python 3.8-3.9 style that returns a dict
-        entrypoints = entry_points().get(group, ())
-        
-    for entrypoint in entrypoints:
+    if sys.version_info >= (3, 10):
+        from importlib.metadata import entry_points
+    else:
+        from pkg_resources import iter_entry_points as entry_points
+
+    for entrypoint in entry_points(group=group):
         entrypoint.load()
         callback = _create_import_hook_from_entrypoint(entrypoint)
         register_post_import_hook(callback, entrypoint.name)