docs: Ensure we also check for python3 sphinx build exec suffixed without a hyphen.
if get_option('documentation')
-cmd_sphinx = find_program('sphinx-build-3')
+cmd_sphinx = find_program('sphinx-build-3', 'sphinx-build3')
cmd_doxygen = find_program('doxygen')
cmd_xsltproc = find_program('xsltproc')
