docs: def namespace_lower(self, namespace):
          """
          Return a copy with only the keys from a given namespace, lower-cased.
  
          The keys in the returned dict will be transformed to lower case after
          filtering, so they can be easily passed as keyword arguments to other
          functions. This is just syntactic sugar for calling
          :meth:`~ConfigLoader.namespace` with
          ``key_transform=lambda key: key.lower()``.
  
          Example::
  
              >>> from configloader import ConfigLoader
              >>> config = ConfigLoader(
              ...     MY_APP_SETTING1='a',
              ...     EXTERNAL_LIB_SETTING1='b',
              ...     EXTERNAL_LIB_SETTING2='c',
              ... )
              >>> config.namespace_lower('EXTERNAL_LIB')
              ConfigLoader({'setting1': 'b', 'setting2': 'c'})
  
          :arg namespace: Common prefix.
  
          :return: New config dict.
          :rtype: :class:`ConfigLoader`
          """
          return self.namespace(namespace, key_transform=lambda key: key.lower())