[mlir][Python] Add a simple PyOpOperand iterator for PyValue uses.

This adds a simple PyOpOperand based on MlirOpOperand, which can has
properties for the owner op and operation number.

This also adds a PyOpOperandIterator that defines methods for __iter__
and __next__ so PyOpOperands can be iterated over using the the
MlirOpOperand C API.

Finally, a uses psuedo-container is added to PyValue so the uses can
generically be iterated.

Depends on D139596

Reviewed By: stellaraccident, jdd

Differential Revision: https://reviews.llvm.org/D139597