replace deprecated assert(cond,msg) with @assert(cond,msg).

Note that these are also technically incorrect usages of assert:

https://mail.python.org/pipermail/python-list/2013-November/660401.html

tl;dr – assertions are to check conditions that should never happen if
the internal logic of the library is correct, not to validate usage.
replace deprecated contains(coll,item) with in(item,coll).
Merge pull request #25 from StefanKarpinski/pull-request/9aaf006b

a couple of deprecation fixes