Split the ``str`` in ``pair_str`` at ``'='``

    Warn if key needs to be stripped

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