Replace deprecated BuildEnvironment.create_index()
Replace deprecated BuildEnvironment.create_index()

This function was deprecated in sphinx 1.6 and removed in 2.0. The
function call is replaced with the recommended replacement according to
https://www.sphinx-doc.org/en/stable/extdev/index.html

Signed-off-by: Sven Schwermer <865f0b31956cbea59f4eabe6d0a49e3dd921de90@disruptive-technologies.com>

rst2pdf/rst2pdf,rst2pdf/rst2pdf