Python Fizz Buzz, Acceptance Unit test
For something so simple, it might be less overkill to use the doctest module.  Write the expected calls and outputs in the docstring, then run python -m doctest -v fizzbuzzmodule.py.  There's no need to write a separate class.

I've written one of the tests to ensure that the function also works when called with decreasing arguments.

As for the function itself, I believe that the code reads better if you write number % 3 == 0 instead of not number % 3.  I would also use a conditional expression, but that's just a matter of personal preference.

def fizzbuzz(number):
    '''
    if number divisible by 3 return 'fizz';
    5, 'buzz'; both, fizzbuzz
    else return number

    &gt;&gt;&gt; fizzbuzz(1)
    1
    &gt;&gt;&gt; fizzbuzz(2)
    2
    &gt;&gt;&gt; fizzbuzz(3)
    'fizz'
    &gt;&gt;&gt; fizzbuzz(4)
    4
    &gt;&gt;&gt; fizzbuzz(5)
    'buzz'
    &gt;&gt;&gt; fizzbuzz(15)
    'fizzbuzz'
    &gt;&gt;&gt; [fizzbuzz(i) for i in range(31, 20, -1)]
    [31, 'fizzbuzz', 29, 28, 'fizz', 26, 'buzz', 'fizz', 23, 22, 'fizz']
    '''
    if number % 3 == 0:
        return 'fizzbuzz' if (number % 5 == 0) else 'fizz'
    if number % 5 == 0:
        return 'buzz'
    return number