Go through source, extracting every key/value pair where the key starts
    with the given prefix.

def get_stripped_prefix(source, prefix):
    """Go through source, extracting every key/value pair where the key starts
    with the given prefix.
    """
    cut = len(prefix)
    return {
        k[cut:]: v for k, v in source.items()
        if k.startswith(prefix)
    }