The contextfilter decorator was deprecated in jinja2 3.0.0, and has been
dropped in 3.1.0. This results in the following warning, and failed
attempts to use filters:
[WARNING]: Skipping plugin (filters.py) as it seems to be invalid:
module 'jinja2' has no attribute 'contextfilter'
This change switches to use the pass_context decorator. The minimum
version of Jinja2 is raised to 3 to ensure pass_context is present.
Change-Id: I649dd6211d3ae72b9539bc44652ef8cf5d579777
fc2292b230