Fix regression in 2.9 involving unsafe Context.get_all() usage

Since commit d67f0fd4cc2a4af08f51f4466150d49da7798729, callers
of Context.get_all() need to make a copy it they're going to
modify the result.

Fixes: d67f0fd4cc2a ("Generalize scoping.  This fixes #603")
1 file changed