Add six and docutils to requirements

[janluke]

This package depends on six and docutils but they are not in requirements. Running it with updated docs dependencies on Python 3.8 I got:

Extension error:
Handler <function run_apidoc at 0x000001E245CEF160> for event 'builder-inited' threw an exception
(exception: No module named 'six')

Adding six to my docs requirements solved the problem of course. But this should not be necessary.

[goerz]

Good point! I'll fix this next time I do any updates on this code...