Add macos-15 to CI test matrix

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.33%. Comparing base (b495de9) to head (5f30972).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #287 +/- ## ========================================== - Coverage 79.34% 79.33% -0.01% ========================================== Files 173 173 Lines 17003 17003 Branches 2309 2309 ========================================== - Hits 13491 13490 -1 Misses 2511 2511 - Partials 1001 1002 +1 ``` | [Flag](https://app.codecov.io/gh/paulsengroup/hictk/pull/287/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paulsengroup) | Coverage Δ | | |---|---|---| | [tests | integration](https://app.codecov.io/gh/paulsengroup/hictk/pull/287/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paulsengroup) | `69.75% <100.00%> (-0.04%)` | :arrow_down: | | [tests | unittests](https://app.codecov.io/gh/paulsengroup/hictk/pull/287/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paulsengroup) | `77.37% <100.00%> (-0.01%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paulsengroup#carryforward-flags-in-the-pull-request-comment) to find out more.

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