Add support for named Other/Proprietary Licenses

[cfarrend]

• I have searched the issues of this repo and believe that this is not a duplicate.
• I have searched the documentation and believe that my question is not covered.

Feature Request

Currently

When you want to use the Other/Proprietary License license type you specify in your pyproject.toml file:

license = "Proprietary"

When uploaded to PyPi, the license shows as the following in the metadata section:

License: Other/Proprietary License (Proprietary)

Desired

We wish to specify our own License name for the Other/Proprietary License instead of the generic "Proprietary" such that we can put in links to our own licenses, or provide the name of the license if not natively supported by PyPi. Some examples are here:

• https://pypi.org/project/aws-kinesis-agg/
• https://pypi.org/project/cdat-lite/
• https://pypi.org/project/pytket-projectq/

Code

Having a dig around the code seems to be around here

poetry/poetry/spdx/__init__.py

Line 48 in ed44342

_licenses["proprietary"] = License("Proprietary", "Proprietary", False, False)

https://github.com/python-poetry/poetry/blob/master/poetry/spdx/license.py

[github-actions]

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