BUG: Series construction with mismatched dt64 data vs td64 dtype
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Fixing this for DataFrame is much harder, so punting for now.
looks fine. this is just a refactor right? doesn't change user facing? > looks fine. this is just a refactor right? doesn't change user facing?

No, this fixes corner cases failing to raise:

```
dtype = np.dtype("m8[ns]")
val = np.datetime64(1234, "ns")

>>> pd.Series([val], dtype=dtype)

>>> pd.Series({0: val}, dtype=dtype)
```

and vice-versa for flipping tdt64/dt64 kk i guess then a whatsnew note 