devel/py-structlog: Update to 17.2.0

While here:

Fix LICENSE
Set architecture neutral

Changes: http://www.structlog.org/en/stable/changelog.html

PR: 219881
Submitted by: Dave Cottlehuber <dch@skunkwerks.at> (maintainer)
Approved by: garga (mentor)
Differential Revision: https://reviews.freebsd.org/D11133