SphinxDirective was added with sphinx 1.8 (2018-09-13).

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220104074649.1712440-1-marcandre.lureau@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>