Fix hash function of PauliOp class (Qiskit/qiskit-aqua#1052)
Fix hash function of PauliOp class (Qiskit/qiskit-aqua#1052)

* Fix hash function of PauliOp class

This change replaces the use of the `id()` function in the `PauliOp`
hash function with the hash of its string representation. This is inline
with the hash function of the `Pauli` class.

This fix is necessary, since prior to this change, a set comparison
including instances of identical `PauliOp`s would fail, because sets
rely on hash tables.

A regression test to ensure the working state after this commit is
included, too.

Credits for spotting and collaborating in fixing the bug go to @Cryoris

* Add releasenote

Co-authored-by: Julien Gacon <761ea7b8820f9c490794378c10a95bac00fe088d@googlemail.com>
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py