fix: prevent RecursionError when pickling Config

On Python 3.8+ with macOS, the multiprocessing module uses 'spawn' by
default which pickles objects. When pickle.load tries to read
__setstate__ before __dict__ is restored, it hits __getattr__ causing
infinite recursion. Adding a special case for 'settings' prevents this.

Closes #2401

gunicorn/config.py

@@ -62,6 +62,8 @@ class Config:
         return "\n".join(lines)
 
     def __getattr__(self, name):
+        if name == "settings":
+            raise AttributeError()
         if name not in self.settings:
             raise AttributeError("No configuration setting for: %s" % name)
         return self.settings[name].get()