bpo-9372: Deprecate several __getitem__ methods (GH-8609)

The __getitem__ methods of DOMEventStream, FileInput,
and FileWrapper classes ignore their 'index' parameters
and return the next item instead.

Lib/fileinput.py

@@ -259,6 +259,13 @@ class FileInput:
             # repeat with next file
 
     def __getitem__(self, i):
+        import warnings
+        warnings.warn(
+            "Support for indexing FileInput objects is deprecated. "
+            "Use iterator protocol instead.",
+            DeprecationWarning,
+            stacklevel=2
+        )
         if i != self.lineno():
             raise RuntimeError("accessing lines out of order")
         try: