Clarify why csv stub has no __version__ attribute (#6398)

Co-authored-by: Akuli <akuviljanen17@gmail.com>

stdlib/_csv.pyi

@@ -1,5 +1,8 @@
 from typing import Any, Iterable, Iterator, List, Protocol, Type, Union
 
+# __version__ is deliberately not defined here or in csv.pyi,
+# as it appears to have been hardcoded at "1.0" for a very long time!
+
 QUOTE_ALL: int
 QUOTE_MINIMAL: int
 QUOTE_NONE: int