CLN: ensure we pass correct type to DTI/TDI shallow_copy
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This seems to have caused a performance regression, see https://pandas.pydata.org/speed/pandas/#index_object.SetDisjoint.time_datetime_difference_disjoint?Cython=0.29.21&python=3.8&x-axis-scale=date

Example from the benchmark:
```
In [5]: N = 10 ** 5
   ...: B = N + 20000
   ...: datetime_left = pd.DatetimeIndex(range(N))
   ...: datetime_right = pd.DatetimeIndex(range(N, B))

In [6]: %timeit datetime_left.difference(datetime_right)
3.27 ms ± 230 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
```

(note I didn't verify it's this commit, but it seems the most related one in the last days)