fix: Make file_urls a private attribute to retain existing behaviors

ocdsextensionregistry/extension_version.py

@@ -25,7 +25,7 @@ def __init__(self, data, file_urls=None):
         self.version = data['Version']
         self.base_url = data['Base URL']
         self.download_url = data['Download URL']
-        self.file_urls = file_urls or {}
+        self._file_urls = file_urls or {}
         self._files = None
         self._metadata = None
         self._schemas = None
@@ -56,8 +56,8 @@ def get_url(self, basename):
         """
         Returns the URL of the file within the extension.
         """
-        if basename in self.file_urls:
-            return self.file_urls[basename]
+        if basename in self._file_urls:
+            return self._file_urls[basename]
         return ''.join([self.base_url, basename])
 
     def remote(self, basename, default=None):