disallow any dunders in the python snippets

[Randle Taylor]

There is a very nice article explaining some of the dangers with eval here:

http://nedbatchelder.com/blog/201206/eval_really_is_dangerous.html

It looks like removing any dunders from the snippets should give an added measure of protection from Bad Things Happening(TM).

Realistically this is not a major concern at this point as all of our users are internal.

[Randle Taylor]

implemented in dev branch