CI: Begin testing on Python 3.12

[effigies]

Numpy 1.26b1 is out, allowing tests on Python 3.12.

Not yet ready to test on numpy 2.0, so leaving the nightly build URLs as-is for now.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.02% :warning:

Comparison is base (5f37398) 92.16% compared to head (a42321f) 92.15%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1247 +/- ## ========================================== - Coverage 92.16% 92.15% -0.02% ========================================== Files 98 98 Lines 12371 12374 +3 Branches 2542 2542 ========================================== + Hits 11402 11403 +1 - Misses 646 648 +2 Partials 323 323 ``` | [Files Changed](https://app.codecov.io/gh/nipy/nibabel/pull/1247?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy) | Coverage Δ | | |---|---|---| | [nibabel/openers.py](https://app.codecov.io/gh/nipy/nibabel/pull/1247?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy#diff-bmliYWJlbC9vcGVuZXJzLnB5) | `93.10% <100.00%> (+0.18%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/nipy/nibabel/pull/1247/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nipy)

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[effigies]

Well, looks like there are numeric issues, or possibly isinstance() or similar checks are now producing different results. Debugging locally seems tricky, as Python 3.12rc1 won't build with pyenv on Ubuntu 22.04. Will try a Docker image when I get a chance.

[effigies]

Not a numeric issue, but gzip: https://github.com/python/cpython/issues/108111