Feature Request: expand B006 and B008 to also cover `ContextVar` defaults

[jakkdl]

from contextvars import ContextVar, copy_context

cv: ContextVar[list[int]] = ContextVar("cv", default=[])

def append_to_cv_list():
    cv.get().append(0)

ctx = copy_context()
ctx.run(append_to_cv_list)
ctx.run(append_to_cv_list)
print(cv.get())

This will print [0, 0] instead of the expected [], very similar to B006 and B008. Can either raise B006 & B008, or just reuse their machinery but raise a new error code.