DEPR/API: Non-ns precision in Index constructors
This deprecates passing dtypes without a precision to DatetimeIndex
and TimedeltaIndex

```python
In [2]: pd.DatetimeIndex(['2000'], dtype='datetime64')
/Users/taugspurger/.virtualenvs/pandas-dev/bin/ipython:1: FutureWarning: Passing in 'datetime64' dtype with no precision is deprecated
and will raise in a future version. Please pass in
'datetime64[ns]' instead.
  #!/Users/taugspurger/Envs/pandas-dev/bin/python3
Out[2]: DatetimeIndex(['2000-01-01'], dtype='datetime64[ns]', freq=None)
```

Closes https://github.com/pandas-dev/pandas/issues/24739
Closes https://github.com/pandas-dev/pandas/issues/24753

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
