Make MultiDictProxy._collect_multiple_keys staticmethod

Author: lafrech

src/webargs/multidictproxy.py

@@ -1,8 +1,8 @@
+from collections.abc import Mapping
+
 from webargs.compat import MARSHMALLOW_VERSION_INFO
 from webargs.core import missing, is_multiple
 
-from collections.abc import Mapping
-
 
 class MultiDictProxy(Mapping):
     """
@@ -18,7 +18,8 @@ def __init__(self, multidict, schema):
         self.data = multidict
         self.multiple_keys = self._collect_multiple_keys(schema)
 
-    def _collect_multiple_keys(self, schema):
+    @staticmethod
+    def _collect_multiple_keys(schema):
         result = set()
         for name, field in schema.fields.items():
             if not is_multiple(field):