Fixing FileInput hook with Pelican version 4.5.0

It is used in rst_include plugin to prefix all rst file with aliases
defined in include.rst

plugins/rst_include/include.py

@@ -11,20 +11,26 @@ is not cumbersome in the daily workflow.
 import os
 from pelican import signals
 from pelican.readers import RstReader
-
+import docutils.io
+import docutils.core
 
 class RstReaderWrapper(RstReader):
     enabled = RstReader.enabled
     file_extensions = ['rst']
 
-    class FileInput(RstReader.FileInput):
+    class FileInput(docutils.io.FileInput):
         def __init__(self, *args, **kwargs):
-            RstReader.FileInput_.__init__(self, *args, **kwargs)
+            super().__init__(*args, **kwargs)
             self.source = RstReaderWrapper.SourceWrapper(self.source)
 
-    # Hook into RstReader
-    RstReader.FileInput_ = RstReader.FileInput
-    RstReader.FileInput = FileInput
+    class Publisher(docutils.core.Publisher):
+        def __init__(self, *args, **kwargs):
+            kwargs['source_class'] = RstReaderWrapper.FileInput
+            super().__init__(*args, **kwargs)
+
+    # Hook into docutils to intercept Publisher construction
+    # in pelican.RstReader._get_publisher and add the `source_class` arg.
+    docutils.core.Publisher = Publisher
 
     class SourceWrapper:
         """