perf: slicify antpair2ind result

[steven-murray]

Description

This simply tries to turn all antpair2ind results into slices. Previously, it would try to turn them into slices to store on the key2ind cache only.

I also restricted pytest to <8.0 for now because of this: pytest-dev/pytest#11868

Motivation and Context

This helps performance, and makes it easier to reason about the output (i.e. it will always be a slice if it possibly can be).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation change (documentation changes only)
• Version change
• Build or continuous integration change

Checklist:

• I have read the contribution guide.
• My code follows the code style of this project.

Bug fix checklist:

• My fix includes a new test that breaks as a result of the bug (if possible).
• All new and existing tests pass.
• I have updated the CHANGELOG.

New feature checklist:

• I have added or updated the docstrings associated with my feature using the numpy docstring format.
• I have updated the tutorial to highlight my new feature (if appropriate).
• I have added tests to cover my new feature.
• All new and existing tests pass.
• I have updated the CHANGELOG.

Breaking change checklist:

• I have updated the docstrings associated with my change using the numpy docstring format.
• I have updated the tutorial to reflect my changes (if appropriate).
• My change includes backwards compatibility and deprecation warnings (if possible).
• I have added tests to cover my changes.
• All new and existing tests pass.
• I have updated the CHANGELOG.

Documentation change checklist:

• Any updated docstrings use the numpy docstring format.
• If this is a significant change to the readme or other docs, I have checked that they are rendered properly on ReadTheDocs. (you may need help to get this branch to build on RTD, just ask!)

Version change checklist:

• I have updated the CHANGELOG to put all the unreleased changes under the new version (leaving the unreleased section empty).
• I have noted any dependency changes since the last version and will update the conda package build accordingly.

Build or continuous integration change checklist:

• If required or optional dependencies have changed (including version numbers), I have updated the readme to reflect this.
• If this is a new CI setup, I have added the associated badge to the readme and to references/make_index.py (if appropriate).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (07a3d7c) 99.92% compared to head (521726b) 99.92%.

Additional details and impacted files

@@            Coverage Diff             @@
##           prep_v3.0    #1387   +/-   ##
==========================================
  Coverage      99.92%   99.92%           
==========================================
  Files             37       37           
  Lines          20967    20966    -1     
==========================================
- Hits           20951    20950    -1     
  Misses            16       16           

Files	Coverage Δ
pyuvdata/uvdata/uvdata.py	100.00% <100.00%> (ø)

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[bhazelton]

Looks good. It looks like this is already reflected in the changelog.