DEPR: undo deprecation of astype(int64) for datetimelike values
Closes #45034
ok so going to back to using .astype here rather than .view, ok with that. ok this is fine. @jorisvandenbossche @jbrockmendel ok here? sure, can merge this for 1.4 and can hammer out better solution for 1.5 thanks @jorisvandenbossche  @meeseeksdev backport 1.4.x Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
git checkout 1.4.x
git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
git cherry-pick -m1 3ac8543dcf3dc2a61945be3bba3855165ef396d7
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
git commit -am 'Backport PR #45449: DEPR: undo deprecation of astype(int64) for datetimelike values'
```

4. Push to a named branch:

```
git push YOURFORK 1.4.x:auto-backport-of-pr-45449-on-1.4.x
```

5. Create a PR against branch 1.4.x, I would have named this PR:

> "Backport PR #45449 on branch 1.4.x (DEPR: undo deprecation of astype(int64) for datetimelike values)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the `Still Needs Manual Backport` label once the PR gets merged.

If these instructions are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                 Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
git checkout 1.4.x
git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
git cherry-pick -m1 3ac8543dcf3dc2a61945be3bba3855165ef396d7
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
git commit -am 'Backport PR #45449: DEPR: undo deprecation of astype(int64) for datetimelike values'
```

4. Push to a named branch:

```
git push YOURFORK 1.4.x:auto-backport-of-pr-45449-on-1.4.x
```

5. Create a PR against branch 1.4.x, I would have named this PR:

> "Backport PR #45449 on branch 1.4.x (DEPR: undo deprecation of astype(int64) for datetimelike values)"

And apply the correct labels and milestones.

Congratulations — you did some good work! Hopefully your backport PR will be tested by the continuous integration and merged soon!

Remember to remove the `Still Needs Manual Backport` label once the PR gets merged.

If these instructions are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                 @jorisvandenbossche if you'd push a manual backport here