Block the installation of Debian's Python packages

[J0WI]

Similar to docker-library/php#542

Edit: in the case of Python it might be a bit risky since a lot of internal tools depends on it. Thoughts?

[tianon]

Yeah, we've discussed this before (although I'm struggling to find references ATM), and the conclusion is that in Python's case this is way too dangerous because there are so many system-level packages/tools that use Python that we can't reasonably block it (unlike PHP).

[yosifkit]

Closing since this isn't really safe to do like in the PHP images.