repoze.sendmail 4.5

• ci: publish to PyPI on release

  Use 'pypi' environment and Trusted Publisher

• Drop pkg_resources namespace support.

• Move package metadata to pyproject.toml.

• Move to src/ layout, with tests/unit/ at top level.

• Add support for Python 3.10 - 3.14.

• Drop support for Python <= 3.9