Acommon preprocessing step in many natural language processing tasks is text normalization, wherein words are converted to lowercase, extraneous whitespace is removed, etc. write a function normalize(text) that returns a normalized version of the input string, in which all words have been
converted to lowercase and are separated by a single space. no leading or trailing whitespace should be present in the output.

> > > normalize("this is an example.")
'this is an example.'
> > > normalize(" extra space ")
'extra space'