bpo-45792: Fix contextvar.Token's intersphinx FQN (GH-29533) (GH-29537)

Since `.. module:: contextvars` sets the module using `.. class:: contextvars.Token`, intersphinx records it as `contextvars.contextvars.Token`.
(cherry picked from commit e501d70)

Co-authored-by: Hynek Schlawack <[email protected]>

Author: miss-islington

Doc/library/contextvars.rst

@@ -94,7 +94,7 @@ Context Variables
           # var.get() would raise a LookupError.
 
 
-.. class:: contextvars.Token
+.. class:: Token
 
    *Token* objects are returned by the :meth:`ContextVar.set` method.
    They can be passed to the :meth:`ContextVar.reset` method to revert