title: Generate time sequence with step 7 seconds
body: <p>How would you generate the following sequence of strings in Python?</p>
  
  <pre><code>00:00:00
  00:00:07
  00:00:14
  00:00:21
  ...
  00:00:49
  00:00:56
  00:01:03
  </code></pre>
  
  <p>The step is 7 seconds. The end is about 03:30:+/-</p>
  
  <p>I would come with solution that uses modular arithmetic (first 1200 to have hours, than 60 to have minutes and the remainder are seconds and the numbers should be converted to strings and "one-place" strings should be prefixed by "0").</p>
  
  <p>Is there some smarter (pythonic) solution with using some helper generators in standard library or list comprehension?</p>