refactor: support python3.12, simplify tests and dependencies

[wpbonelli]

Add support for Python 3.12. There are a few sticking points

• pytest-virtualenv is unmaintained and not compatible with 3.12
  • previously used to test flopy.mf6.utils.generate_classes
  • solution: fall back to testing with virtualenv directly
• pyzmq (required by jupyter/ipykernel) wheel fails to build
  • possibly related: unable to build on mac m1 max zeromq/pyzmq#1907, Add official support for Python 3.12 kedro-org/kedro#3287
  • solution: use micromamba with conda-forge distribution for tests where necessary
  • commit.yml smoke tests, mf6.yml, rtd.yml, optional.yml, release.yml still use python/pypi

Also

• reduce duplication in CI workflows
• minor cleanup in autotest/conftest.py
• add pip to etc/environment.yml
• replace bmipy with xmipy in etc/environment.yml
• add IPRN value where missing in some example input files (unblocks failing CI in other PRs)
• add generation test marker (code-generation utilities) to pytest.ini

[codecov]

Codecov Report

Merging #1999 (b9feade) into develop (696a209) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           develop   #1999   +/-   ##
=======================================
  Coverage     72.7%   72.7%           
=======================================
  Files          258     258           
  Lines        58824   58824           
=======================================
  Hits         42794   42794           
  Misses       16030   16030