feat(index): ➖ arithmetic subtraction

[cmp0xff]

It belongs to the series of changes suggested by #1347, although not in the sense of a Pandas Series.

• Tests added: Please use assert_type() to assert the type of any return value

[Dr-Irv]

Looks pretty good

[Dr-Irv]

The only thing I'm worried about here is that we don't necessarily have a complete set of tests for subtracting timestamps, timedeltas, series, indexes, lists of such from DatetimeIndex and TimedeltaIndex, or from a plain Index where we detect an error. There are some that exist and are scattered around the tests.

Maybe we do a separate PR for that? Or handle here?

[cmp0xff]

I understand that a set of systematic tests is desirable whenever the corresponding code is modified. In this case, however, I would personally prefer to develop these tests in a separate pull request or issue.

It has been quite exhausting to work out the timestamp and timedelta subtractions; the challenge may trace back to the fact that the subtraction here is not closed in the algebraic sense.

If needed, I can also revert the changes in DatetimeIndex and TimedeltaIndex, so that they are unmodified in this PR.

[Dr-Irv]

thanks @cmp0xff

[Dr-Irv]

I understand that a set of systematic tests is desirable whenever the corresponding code is modified. In this case, however, I would personally prefer to develop these tests in a separate pull request or issue.

That's fine.