Make DTI[tz]._values and Series[tz]._values return DTA
broken off of #24024, cc @jreback @TomAugspurger 

I think the edits in core.dtypes.concat are unrelated, but they are correct regardless and easy to trim a few more lines off the diff.

The edit in tests.indexing.test_coercion was needed during troubleshooting, decided to keep it.