support.plugin: abominable workaround for Python 3.8-3.9.

plugin: fix type error setting `requirement.marker`.

support.plugin: add python 3.8-3.9 support.

manual: community: add guidance re: meetings.

cli: add support for passing command-line arguments to the REPL and script environments

manual: purchase: add a section on shipping timelines.

manual: community: add a section on weekly meetings.

manual: make the call-to-action announcement green.

manual: clearly mark preview builds of documentation in the header.

manual: ensure the canonical relation is set. Otherwise we'll start getting issues with PR builds of the docs whenever those get indexed!