Set up readline with given parameters, saving old state.
This function is safe to call if readline is absent; it will
silently do nothing.
fn is a filename, relative to the home directory, for the history file.
completer is a completion callable (see readline module documentation),
or as a convenience, True may be passed for an interactive Python
delimns is a string containing characters that delimit completion words.
passing None will use teh default set.
environ is the environment in which to look up variables such as the
lines is the number of lines of history to keep, or -1 for unlimited.