There is a backward compatibility issue with changeset d5ef330bac50
that enables tab-completion in the interactive interpreter by default.

When a user is not aware of this new feature and has been implementing
up to now her/his PYTHONSTARTUP file with the first example given in
python 3.3 documentation at

http://docs.python.org/3/library/readline.html?highlight=readline#example

then each new interactive python 3.4 session reads (appending them)
both history files which are saved on exit. Thus those files double
their size after each interactive python 3.4 session and pretty soon
become few mega bytes long with a python start up time that becomes
quite noticeable.