graphics/qgis: upgrade 3.6.2 -> 3.8.2

• Force clang version 9.x from ports
• Update several Python dependencies
• Add devel/py-geojson, devel/py-nbformat, math/py-networkx, graphics/py-pillow, graphics/py-scikit-image, and devel/py-shapely for Python extension
• Add -DCMAKE_POLICY_DEFAULT_CMP0056:STRING=NEW

PR: 240124
Submitted by: Rainer Hurling <rhurlin@gwdg.de> (maintainer)
Relnotes: https://www.qgis.org/en/site/forusers/visualchangelog38/index.html