Issues with pyproject.toml

Since the recent pytest v9 release, PEFT's pyproject.toml does not work anymore. This is because it mixes both new and old format new formats (ini_options), which is no longer allowed.

This PR fixes the issue by switching fully to the new format. Note that if a pytest version < 9.0.0 is used, the added pytest options will be ignored. Thus please update to 9.0.0+.

Context: https://github.com/pytest-dev/pytest/releases/tag/9.0.0

Example of failing CI: https://github.com/huggingface/peft/actions/runs/19218760387/job/54932519290#step:5:14

Issues with unittest.skip

Since pytest v9, unittest.skip raises an error instead of just skipping the corresponding test (see these errors). As of writing this, it is unclear if this will be fixed or not:

pytest-dev/pytest#13895

To avoid the error, we now use pytest.mark.skip insted of unittest.skip / unittest.skipUnless.

Tangential changes

1. Remove a few inheritances from unittest.TestCase that were unnecessary
2. Skip the whole test_gpu_examples.py module if no accelerator is available (faster than checking each class/function individually)