Exercise: word frequency

Write a function word_count(text) that returns a dict: the keys are the words of the string text in lower case, and the values are how many times each word occurs.

• word_count("the cat the") → {'the': 2, 'cat': 1}
• word_count("Go go GO") → {'go': 3}
• word_count("") → {}

Constraints: words consist of Latin letters, with no punctuation; there may be several spaces between words, as well as at the start and the end of the string; words that differ only in letter case count as one word.

Do not add extra imports unless the task says so explicitly.