Poetry: Support "Other/Proprietary License"

[thmo]

• [x] I have searched the issues of this repo and believe that this is not a duplicate.

Issue

For an internal project, I'd like to use Other/Proprietary License as the license (which seems to be a valid choice e.g. according to https://pypi.org/classifiers/), but that doesn't seem to be supported by poetry.

[HolgerPeters]

@sdispater I would like to create a PR for this if there are no objections

[Nateyo]

+1

A custom/proprietary license field would be great.

[sdispater]

You can now use the special proprietary license identifier to specify that the package uses a proprietary license.