Fixed the type declaration for all_errors in ftplib.pyi. It is supposed to be a variable, not a type alias. (#5271)

Co-authored-by: Eric Traut <[email protected]>

Author: erictraut

stdlib/ftplib.pyi

@@ -22,7 +22,7 @@ class error_temp(Error): ...
 class error_perm(Error): ...
 class error_proto(Error): ...
 
-all_errors = Tuple[Type[Exception], ...]
+all_errors: Tuple[Type[Exception], ...]
 
 class FTP:
     debugging: int