Make Python MLIR Operation not iterable

The current behavior is conveniently allowing to iterate on the regions of an operation
implicitly by exposing an operation as Iterable. However this is also error prone and
code that may intend to iterate on the results or the operands could end up "working"
apparently instead of throwing a runtime error.
The lack of static type checking in Python contributes to the ambiguity here, it seems
safer to not do this and require and explicit qualification to iterate (`op.results`, `op.regions`, ...).

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D111697