BUG: unexpected assign by a single-element list (GH19474)
- [x] closes #19474
- [x] tests added / passed (some failed independently of my changes)
- [x] passes `git diff master --name-only -- "*.py" | flake8`
- [x] whatsnew entry

I thought it is a straightforward way to evaluate all of `df.loc[0, ['A']] = ['X']`, `df.loc[0, ['A', 'B']] = ['X', 'Y']`, ...(more-columns case) the same way in `else` block at line 590 of pandas/core/indexing.py.

Please let me know bad or ungrammatical points.