graphics/py-exiv2: Update 0.18.1 => 0.19.0, take maintainership

Changelog:
1/ Add exiv2.XmpParser.encode() to Python interface.
2/ Fix crash when getting strings of malformed values.
3/ Python 3.15 compatibility.
https://github.com/jim-easterbrook/python-exiv2/releases/tag/0.19.0
https://github.com/jim-easterbrook/python-exiv2/blob/0.19.0/CHANGELOG.txt

• Add STRIP_CMD in post-install.

PR: 297603
Sponsored by: UNIS Labs
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>
MFH: 2026Q3

(cherry picked from commit 59d9d76281846c9e3427ebecd3da6b28aea5f631)