This avoids a CRITICAL error from rstcheck.

    CRITICAL:rstcheck_core.checker:An `AttributeError` error occured.
    This is most propably due to a code block directive (code/code-
    block/sourcecode) without a specified language. This may result in a
    false negative for source: 'README.rst'. See
    https://rstcheck-core.readthedocs.io/en/latest/faq/#code-blocks-without-language-sphinx
    for more information.