Allow building against HDF5 1.12

[Anthchirp]

HDF5 1.12 has been out for nearly two years (February 2020), so I think we can start looking into running on it and see what breaks.

I welcome comments of caution 😄

[graeme-winter]

This absolutely needs trying out, to find out what the impact is. Will investigate.

[Anthchirp]

merging in main, so that investigations would cover current numpy+hdf5 versions

[codecov]

Codecov Report

Merging #1973 (ff55856) into main (e611c76) will not change coverage.
The diff coverage is n/a.

❗ Current head ff55856 differs from pull request most recent head 8db3ab3. Consider uploading reports for the commit 8db3ab3 to get more accurate results

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[ndevenish]

This will get better testing in the main repo, and in any case the conda-forge package appears to be using HDF5 1.12 already.

[ndevenish]

Correction: I had thought this was the dxtbx CI, in which case the merge title would have been correct. I apologies for the somewhat confusing mistake.