fix: replace removed distutils.spawn with shutil.which (Python 3.12+)

[alongd]

Problem

distutils was removed from the Python standard library in 3.12, so arc/main.py's
from distutils.spawn import find_executable raises ModuleNotFoundError on Python 3.14.
It only appears to work in environments where setuptools happens to install its distutils
compatibility shim — a fresh CI env without that shim fails.

Because arc.main is imported transitively by most of ARC (and by downstream consumers like
T3), this breaks at import/collection time. Concretely, the T3 test suite currently dies with:

ImportError while importing test module '.../tests/test_common.py'.
E   ModuleNotFoundError: No module named 'distutils'

(chain: t3.common → arc.species.species → arc.main → from distutils.spawn import find_executable)

Fix

Replace the single stdlib usage with shutil.which, the documented drop-in replacement for
distutils.spawn.find_executable — both return the path to the executable found on PATH, or
None. shutil is already imported in arc/main.py, so no new import is added.

The 7 affected call sites all look up ESS executables on PATH (g03/g09/g16/qchem/orca/
molpro/terachem), and the downstream truthiness checks (if g03 or g09 or g16: …) are unchanged.

Scope / safety

• This is the only distutils usage anywhere in arc/*.py, and there are no other
  removed-stdlib imports — this single change makes the ARC import graph Python-3.12+/3.14 clean.
• ARC's own arc.settings.settings.find_executable(env_name, ...) is a different function
  (it locates the python interpreter of a named conda env) and is intentionally left untouched;
  arc/job/adapters/ase_adapter.py imports that settings function, not the stdlib one.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates ARC’s ESS executable discovery to be compatible with Python 3.12+ by removing the deprecated/removed distutils dependency and using the standard-library replacement.

Changes:

• Removed from distutils.spawn import find_executable from arc/main.py.
• Replaced ESS executable lookups (g03/g09/g16/qchem/orca/molpro/terachem) with shutil.which().

Comments suppressed due to low confidence (1)

arc/main.py:812

• The else: that logs "Did not find ESS on the local machine" is currently attached to if diagnostics: (inside the if any(...) block). This causes a misleading log message when ESS executables are found but diagnostics is False. The else should instead pair with if any(...) (and optionally simplify the any([...]) expression).

            terachem = shutil.which('terachem')
            if terachem:
                self.ess_settings['molpro'] = ['local']
            if any([val for val in self.ess_settings.values()]):
                if diagnostics:

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.10%. Comparing base (e7f2c98) to head (c62bdbb).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #908      +/-   ##
==========================================
- Coverage   63.15%   63.10%   -0.06%     
==========================================
  Files         114      114              
  Lines       38178    38177       -1     
  Branches     9990     9990              
==========================================
- Hits        24113    24090      -23     
- Misses      11183    11199      +16     
- Partials     2882     2888       +6     

Flag	Coverage Δ
functionaltests	63.10% <ø> (-0.06%)	⬇️
unittests	63.10% <ø> (-0.06%)	⬇️

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