Adding license classifier

[RobPasMue]

Confirm this is a feature request for the Python library and not the underlying OpenAI API.

• This is a feature request for the Python library

Describe the feature or improvement you're requesting

See #826

It is important that Python packages have proper classifiers added. That way we can make use of PyPI's metadata to detect the license of the package easily.

Additional context

No response

[RobPasMue]

Closed by #826