sip-sdist ignores lists in metadata

[Dmitry Shachnev]

Hi Phil! I have a small bug report.

sip documentation on metadata section [1] says:

> If a value is a list of individual values then the key will be repeated in
> the generated meta-data for each of those values.

https://www.riverbankcomputing.com/static/Docs/sip/pyproject_toml.html#tool-sip-metadata-section

However, I tried putting some classifiers into pyproject.toml but they were
ignored:

[tool.sip.metadata]
...
classifier = [
  "Development Status :: 5 - Production/Stable",
  "Intended Audience :: Developers"
]

This is probably because of this check:

https://riverbankcomputing.com/hg/sip/file/5.4.0/sipbuild/pyproject.py#l96

--
Dmitry Shachnev