support & test Python 3.8-3.12 instead of 3.9-3.11 + bump CI pins

[asumagic]

What does this PR do?

#2452 warns that a lot of stuff still seems to rely on Python 3.8. We have informally been fixing some issues with this version so we might as well make it a proper tested target instead of relying on reports. Thus:

• Add 3.8 to the test matrix in CI
• Add 3.8 back as an officially supported version, but deprecate it
• Bump from Python 3.7 to 3.8 in setup.py as that should probably have been updated a while ago

Python 3.12 has decent support across the board (as far as I can tell, k2, PyTorch and Numba support it nowadays). Thus:

• Add 3.12 to the test matrix in CI
• Add 3.12 as an officially supported version
• Bump the pinned k2 version in CI along the way
• Bump the pinned kaldilm version (1.15 not available for Python 3.12 apparently)

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• Did you make sure to update the documentation with your changes? (if necessary)
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PR review

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[pplantinga]

I think this is a good change, it's good to test all the versions that we support if possible. My main concern is that it may close to double our CI minutes usage each month, is this something we have the capacity for or will it cause us to run out during the month @mravanelli ?

[pplantinga]

One alternative could be to test 3.8, 3.10, 3.12

[asumagic]

Ah, yeah, sorry, I hadn't noticed the notification on this PR. I've changed the tested versions to 3.8 and 3.12 if you think that makes sense.

[pplantinga]

Good change!