Use relative import for utils in test_pythoncapi_compat

[different-name]

Fix for the following test error:

============================= test session starts ==============================
platform linux -- Python 3.13.12, pytest-9.0.2, pluggy-1.6.0
rootdir: /build/srctools-2.6.2
configfile: pyproject.toml
plugins: regressions-2.9.1, datadir-1.8.0
collecting ... ^Mcollecting 1585 items / 1 error                                                ^Mcollected 3619 items / 1 error                                                 

==================================== ERRORS ====================================
____ ERROR collecting src/pythoncapi-compat/tests/test_pythoncapi_compat.py ____
ImportError while importing test module '/build/srctools-2.6.2/src/pythoncapi-compat/tests/test_pythoncapi_compat.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/nix/store/m1fw8l8y9ycxh5dzispbb7cwl6rra14l-python3-3.13.12/lib/python3.13/importlib/__init__.py:88: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
src/pythoncapi-compat/tests/test_pythoncapi_compat.py:24: in <module>
    from utils import run_command, command_stdout
E   ModuleNotFoundError: No module named 'utils'
=========================== short test summary info ============================
ERROR src/pythoncapi-compat/tests/test_pythoncapi_compat.py
!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!
=============================== 1 error in 0.82s ===============================

Not sure why this is only an issue now, however this seems to be the correct approach