Python: Undeprecate `proclaim`

[RyanGlScott]

#1644 adds support for llvm_assert, which behaves like llvm_precondition or llvm_postcondition depending on where it is used in a specification. We would like to have a Python counterpart to llvm_assert, and in fact, we already have one in the form of the proclaim function. proclaim is currently marked as deprecated, but given that we want to support something like it going forward, it makes sense to remove the deprecation warning and allow users to call it without warnings.

[smithnormform]

It might also be nice to have a proclaim_f method like precondition_f and postcondition_f.