Error building from source

[rcomer]

🐛 Bug Report

Cannot currently build cf-units from source on Met Office machine with python 3.12. Everything is fine if I pin python=3.11.

How to Reproduce

Steps to reproduce the behaviour:

1. Create conda environment from requirements/cf-units.yml
2. python -m pip install -v .

Error:

  In file included from cf_units/_udunits2.c:20448:0:
  /.../conda_envs/cf-units-dev/include/python3.12/internal/pycore_frame.h: In function '_PyFrame_Initialize':
  /.../conda_envs/cf-units-dev/include/python3.12/internal/pycore_frame.h:132:5: error: 'for' loop initial declarations are only allowed in C99 mode
       for (int i = null_locals_from; i < code->co_nlocalsplus; i++) {
       ^
  /.../conda_envs/cf-units-dev/include/python3.12/internal/pycore_frame.h:132:5: note: use option -std=c99 or -std=gnu99 to compile your code
  error: command '/usr/bin/gcc' failed with exit code 1
  error: subprocess-exited-with-error

Expected Behaviour

cf-units builds without error

Environment

• OS & Version: RHEL7
• cf-units Version: main

Additional Context

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[bjlittle]

@rcomer Thanks for raising this issue.

I suspect that we need to make some minor tweaks to cf-units to use cython 3.x and build correctly for py312.

I'm keen to make a fix for this and force a release of cf-units soon.

[rcomer]

I cannot reproduce the problem on Ubuntu 22.04.