Starting in Python 3.8, the function node definition's `lineno` is changed to index its `def ...` line rather than the first line where its decorators start. This causes inconsistent line numbers across Python versions for the line reported by Flake8.

We can use the decorator node location instead, which provides a consistent location, and makes sense because this hits on decorators.

* Prefer `extend-select` and `extend-ignore` for configuring opinionated warnings (`B9`)
* Add deprecation note for Bugbear's internal handling of whether or not to emit `B9` codes
* Add an example for `extend-immutable-call` specification

* The code for Bugbear's built-in filtering for opinionated warnings predates the addition of `extend-select` to flake8 (`v4.0`) so it's not part of the check for explicit specification of `B9` codes.
* Switch from `Mock` to `argparse.Namespace` for specifying options to tests to match the incoming type from `flake8` and avoid mocking side-effects.

Co-authored-by: Cooper Ry Lees <me@cooperlees.com>