Consolidate package metadata in pyproject.toml, use setuptools_scm and add python 3.12

[ericpre]

Progress of the PR

• Consolidate package metadata in pyproject.toml,
• Use setuptools_scm to set version in package at build time,
• Add python 3.12,
• [n/a] docstring updated (if appropriate),
• update user guide (releasing_guide.md),
• added tests (package and test workflow),
• added line to CHANGES.rst,
• ready for review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9eceecf) 94.41% compared to head (50c8302) 87.38%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #14      +/-   ##
==========================================
- Coverage   94.41%   87.38%   -7.04%     
==========================================
  Files          12        7       -5     
  Lines         591      325     -266     
  Branches        0       72      +72     
==========================================
- Hits          558      284     -274     
+ Misses         33       25       -8     
- Partials        0       16      +16     

Files	Coverage Δ
holospy/__init__.py	85.71% <100.00%> (-3.18%)	⬇️

... and 6 files with indirect coverage changes

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