docs: The CPython `random` module source code ends up in the docs

[microbit-carlos]

Technically two modules seem to be added, posix and random, but the posix link is a 404:

• https://microbit-micropython.readthedocs.io/en/v1.0.1/_modules/index.html
• https://microbit-micropython.readthedocs.io/en/v1.0.1/_modules/random.html

Not sure how/why Sphinx is picking these modules. They are not really linked anywhere (that I can tell), so not a big deal, but it does show up in the search.