fix: use valid license identifier (SPDX)

[jooola]

Fixes #513

********************************************************************************
Please consider removing the following classifiers in favor of a SPDX license expression:

License :: OSI Approved :: MIT License

See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
********************************************************************************

See https://github.com/hetznercloud/hcloud-python/actions/runs/15873016015/job/44753884445#step:5:24

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.43%. Comparing base (bcbdd7d) to head (de7138d).
⚠️ Report is 41 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #514   +/-   ##
=======================================
  Coverage   93.43%   93.43%           
=======================================
  Files          64       64           
  Lines        3048     3048           
=======================================
  Hits         2848     2848           
  Misses        200      200           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.