[matthias-bach-by]

Summary

• Add a default tox packaging environment on Python 3.11 that builds distributions and runs Twine with Python warnings treated as errors.
• Use the BSD-3-Clause SPDX expression and remove the deprecated BSD license classifier; configure setuptools-scm to avoid warnings during strict builds.
• Reuse the tox packaging environment in CI and fetch full Git history in test and publish workflows.
• Document the local packaging check.

Validation

• tox -e packaging passes.
• tox -e 3.11 passes (41 tests).
• A deprecated license-classifier probe fails as expected under the strict warning setting.
• Full local tox ran packaging successfully; test environments 3.11–3.13 collided on the local devpi server port.

[coveralls]

coverage: 100.0%. remained the same — strict-packaging-checks into master