BUG: avoid StringArray.__setitem__ to mutate the value being set as side-effect

[jorisvandenbossche]

Encountered this while working on #51082, and this can be done as independent fix.

In [5]: arr = pd.array(["a", "b", "c"], dtype="string")

In [6]: value = np.array(["A", None])

In [7]: arr[[0, 1]] = value

In [8]: value
Out[8]: array(['A', <NA>], dtype=object)  # <-- value got mutated !!

I think in general the value that is being set should never be mutated during the setitem operation.

• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

[mroeschke]

Looks good. I imagine this fix should have a whatsnew note?

[mroeschke]

Thanks @jorisvandenbossche