Add changelog URL to package metadata (#6770)

When Renovate bot creates an automated pull request to update Poetry, it
can automatically include the link to changelog so it's easier to find
out what changed.

The URL name has to be one of: 'changelog', 'change log', 'changes',
'release notes', 'news', "what's new" (case insensitive)


https://github.com/renovatebot/renovate/blob/c3a87b687ed44a1a100e0e4f91b81e22b7c32482/lib/modules/datasource/pypi/index.ts#L130-L140



Co-authored-by: Bjorn Neergaard <bjorn@neersighted.com>