It is already available:
>>> import pydoc
>>> pydoc.cram('This sentence is too long to fit the space I have made available', 28)
'This sentenc...ade available'

def cram(text, maxlen):
    """Omit part of a string if needed to make it fit in a maximum length."""
    if len(text) > maxlen:
        pre = max(0, (maxlen-3)//2)
        post = max(0, maxlen-3-pre)
        return text[:pre] + '...' + text[len(text)-post:]
    return text

It could be documented in place, or moved and imported into pydoc. I am +0 at the moment.