TYP: Update timestamps.pyi
Thanks @hauntsaninja! I assume you use the not-yet-released mypy 0.970? When I run `python -m mypy.stubtest pandas --ignore-missing-stub --concise --mypy-config-file pyproject.toml`

I only get
```sh
error: not checking stubs due to mypy build errors:
pandas/core/indexes/base.py:7138: error: Cannot determine type of "argmin"  [has-type]
pandas/core/indexes/base.py:7150: error: Cannot determine type of "argmax"  [has-type]
pandas/core/window/rolling.py:1842: error: Unused "type: ignore" comment
pandas/core/window/rolling.py:1848: error: Unused "type: ignore" comment
```

Seems there are some type errors:
https://github.com/pandas-dev/pandas/runs/7373010497?check_suite_focus=true I used mypy==0.960 for this, but did a couple unconventional things to get things running :-) I'll file an issue for running stubtest in CI and help get it integrated properly.

Btw, what is the relationship between the stubs in pandas-dev/pandas.git and the stubs in pandas-dev/pandas-stubs.git? > Btw, what is the relationship between the stubs in pandas-dev/pandas.git and the stubs in pandas-dev/pandas-stubs.git?

Ideally, they should be the same (and I hope that in the future they are the same). In many cases, pandas and pandas-stubs are/will hopefully be aligned but in certain cases, it is impossible(?) to find a "good" annotation. For pandas-stubs, "good" roughly means it works for users. For pandas, "good" roughly means what is technically possible (e.g., `pd.Timestamp(float("NaN"))` returns `NaTType` or another difficult case is `__getitem__` #46616). Thank you @hauntsaninja!