import-checker: allow symbol imports from typing module
As we add typing annotations, we'll want to use a lot of symbols from
the `typing` module. Typing `typing` all the time will be annoying. Let's
allow symbol imports from this module.
While I was here, I changed some comments from "whitelist" to "allow list"
as the former is non-inclusive terminology.
Differential Revision: https://phab.mercurial-scm.org/D12365
pycompat: remove json.loads polyfill for Python 3.5
We no longer support Python 3.5 so this can be deleted.
Differential Revision: https://phab.mercurial-scm.org/D12364