REF: expressions bug in Block.where

[jbrockmendel]

• closes API/DEPR: int downcasting in DataFrame.where #44597
• tests added / passed
• Ensure all linting tests pass, see here for how to run them
• whatsnew entry

Probably needs to be done as a deprecation if done for real, but ATM we have no tests that are affected by this (update: we do, just not with my local platform/versions). One would be:

arr = np.arange(6).astype(np.int8).reshape(3, 2)
df = pd.DataFrame(arr)

mask = np.array([[False, True], [False, False], [False, True]])

res = df.where(~mask, 257)

on master, res.dtypes are int8 and int16. In this PR, res.dtypes are both int16.