Revert a change removing the default adapter argument on SpecLoaderAdapter.__init__()

Author: whitphx

Lib/importlib/resources/_adapters.py

@@ -9,7 +9,7 @@ class SpecLoaderAdapter:
     Adapt a package spec to adapt the underlying loader.
     """
 
-    def __init__(self, spec, adapter):
+    def __init__(self, spec, adapter=lambda spec: spec.loader):
         self.spec = spec
         self.loader = adapter(spec)