MNT: CPython 3.12 compatibility (progress tracker)

[neutrinoceros]

Opening this to keep track of exactly where we're at and what's left to do:

• BLD: add wheels for CPython 3.12 #4625
• TST: start testing nightlies on CPython 3.12 #4626
• BLD: require Cython 3.0.3 to fix known issues on CPython 3.12 #4687
• future: switch tests with full dependencies to 3.12 (and remove the simple job introduced in BLD: require Cython 3.0.3 to fix known issues on CPython 3.12 #4687). This last step should be done when all optional dependencies with significant build time have cp312 wheels on PyPI. In particular, look out for
  • h5py
  • cartopy
  • netCDF4
  • aiohttp

[neutrinoceros]

cartopy doesn't ship cp312 wheels yet but the latest version builds fine on Python 3.12 so it's not a hard requirement to close this issue