special exception for rst2pdf.pdfbuilder, we know it is safe

sphinx/application.py

@@ -446,6 +446,9 @@ class Sphinx(object):
                     'version.' % (extension, err))
         if ext_meta is None:
             ext_meta = {}
+            # special-case for compatibility
+            if extension == 'rst2pdf.pdfbuilder':
+                ext_meta = {'parallel_read_safe': True}
         try:
             if not ext_meta.get('version'):
                 ext_meta['version'] = 'unknown version'