[mlir][python] Hook up PyRegionList.__iter__ to PyRegionIterator

This fixes a -Wunused-member-function warning, at the moment
`PyRegionIterator` is never constructed by anything (the only use was
removed in D111697), and iterating over region lists is just falling
back to a generic python iterator object.

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D150244