DEPR: move NumericIndexes into base Index, part 1

[topper-123]

A part of the changes in pandas 2.0 will be that the current numeric indexes (Int64Index, Uint64Index & Float64Index) will be removed and their functionality added to the base Index. In addition, the base index should be able to handle all numpy numeric dtypes (int8, int16 tc.), not just the current int64, uint64 & float64.

This PR makes progress towards that. In this PR I:

• remove Int64Index, Uint64Index & Float64Index from the top namespace
• Make instantiation of Index with numpy numeric dtypes return a NumericIndex with the given dtype rather thanconverting to the old 64 bit only numeric index types, making all numeric dtypes available for indexes.

This is just the first part of the complete changes planned. In follow-ups PRs I will:

• remove Int64Index, Uint64Index & Float64Index from the code base entirely (also internally)
• Move the functionality of NumericIndexinto the base Index and remove NumericIndex
• Update docs to reflect the above changes

Notable changes from this PR:

One notable change from the PR is that because we now have all the numeric dtypes available for indexes, indexes created from DateTimeIndex (day, year etc.) will now be in int32, where previously they were forced to int64 (because that was the only index integer dtype available).

Using int32 is the more correct way, because the underlying DateTimeArray returns 32bit arrays. An example of this:

>>> import pandas as pd
>>> 
>>> x = pd.date_range(start='1/1/2018', end='1/08/2018')
>>> x.array.day
array([1, 2, 3, 4, 5, 6, 7, 8], dtype=int32)  # 32bit, current and will not be changed in this series of PRs
>>> x.day
Int64Index([1, 2, 3, 4, 5, 6, 7, 8], dtype='int64')  # before this PR
NumericIndex([1, 2, 3, 4, 5, 6, 7, 8], dtype='int32')  # after this PR
Index([1, 2, 3, 4, 5, 6, 7, 8], dtype='int32')  # after follow-up PRs

xref #42717, #41272

[jbrockmendel]

looks like a couple of sparse tests are failing

[topper-123]

Yeah, test suite passed locally, trying to figure out what's the difference. Probably I need to install some package locally, so tests are skipped locally.

UPDATE: everything passes locally still, so I'm a bit stomped why this fails in Github. I'll look again tomorrow.

[jbrockmendel]

comment about dtype?

[topper-123]

Closing in favor of #49560.