Use SPDX string for project.license in pyproject template

[nthbotast]

Summary

Fixes the setuptools deprecation warning described in #254 by updating the generated pyproject.toml template:

• change project.license from TOML table to SPDX string ("MIT")
• remove deprecated MIT license classifier from project.classifiers

This follows the packaging guidance referenced in the issue and avoids the upcoming setuptools breakage window for deprecated license metadata.

Validation

• Template file updated: {{ cookiecutter.__dirname }}/pyproject.toml
• Attempted to run tests in a fresh venv; test collection currently fails in this environment due to an upstream dependency compatibility warning (requests/urllib3 on Python 3.14), unrelated to this metadata-only change.

[tekktrik]

Thanks!

[tekktrik]

Merging despite failing - see #256 for more information