drop python 3.8 support

[rkingsbury]

Drop support for python 3.8 as it reached end of life some time ago. Add python 3.12 to the CI testing in its place.

EDIT: cannot easily add python 3.12 at this time because montydb does not support it. Will address separately / later.

Closes #917

[codecov]

Codecov Report

Attention: Patch coverage is 97.61905% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 81.33%. Comparing base (2559136) to head (1fc0e33).

❗ Current head 1fc0e33 differs from pull request most recent head 3f2fdb6

Please upload reports for the commit 3f2fdb6 to get more accurate results.

Files	Patch %	Lines
src/maggma/stores/open_data.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #951      +/-   ##
==========================================
+ Coverage   81.26%   81.33%   +0.07%     
==========================================
  Files          46       46              
  Lines        3971     3971              
==========================================
+ Hits         3227     3230       +3     
+ Misses        744      741       -3     

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