Typing of Dataset

[headtr1ck]

• Closes Start using | instead of Union or Optional when typing #5945
• Tests added

Feel free to wait with merging after the 2022.05.0.dev0 release :)

• This PR adds typing to all Dataset methods (only the ones in Dataset directly and DataWithCoords).
• On the way it fixes several typing issues and adds small code changes for more consistent typing (hopefully does not break anything).
• All test_dataset tests are now typed.
• As usual there are still some mypy bugs open that prevent all typing changes to be final (especially bound TypeVar messes with argument type resolving python/mypy#12846). I choose to remove the TypeVars in these occasions to prevent false positives on the user side.
• Sorry for updating so many files at once, mostly these are the changes of adding from __future__ import annotations and automatic fixes from pyupdate.

If anyone has some typing expertise:
I think that the way align is typed is wrong. The current implementation only works for aligning a sequence of objects of the same type, but not mixed DataArrays and Datasets. In some placed I had to add some "#type: ignore"s...
I should probably open an issue for that.

[headtr1ck]

It seems that the reductions doc outcome differs on my machine.
Is it possible to generate it in CI?

[dcherian]

@Illviljan had similar int32 vs int64 issues. I guess you're on Windows too?

CI seems like a good idea. I'll fix it manually for now.

Also thanks for this PR. Amazing work!

[headtr1ck]

@Illviljan had similar int32 vs int64 issues. I guess you're on Windows too?

Yes this was on windows.

[max-sixty]

Great work @headtr1ck !!