docs: def split_mapping(pair_str):
      """Split the ``str`` in ``pair_str`` at ``'='``
  
      Warn if key needs to be stripped
      """
      orig_key, value = pair_str.split("=", 1)
      key = orig_key.strip()
      if key != orig_key:
          warnings.warn(
              "Mapping key {} has leading or trailing space".format(repr(orig_key)),
              LeadingTrailingSpaceInKey,
          )
      return key, value