[3.12] gh-113028: Correctly memoize str in pickle when escapes added (GH-113436) (GH-113448)

This fixes a divergence between the Python and C implementations of pickle
for protocol 0, such that it pickle.py fails to re-use the first pickled
representation of strings involving characters that have to be escaped.
(cherry picked from commit 08398631a0298dcf785ee7bd0e26c7844823ce59)

Co-authored-by: Jeff Allen <ja.py@farowl.co.uk>