Dtype checking in reshape.merge

[jbrockmendel]

There is a check (equivalent to)

elif (issubclass(lk.dtype.type, (np.integer, np.timedelta64, np.datetime64)) and
      issubclass(rk.dtype.type, (np.integer, np.timedelta64, np.datetime64))):

which I'm told is incorrect:

this is not right, these should be in separate branches here, the datetime / timedeltas can be in a single elif clause.

#23917 (comment)