[3.12] gh-108303: Move all certificates to `Lib/test/certdata/` (GH-109489) (#109682)

* gh-108303: Move all certificates to `Lib/test/certdata/` (GH-109489)
(cherry picked from commit e57ecf6bbc59f999d27b125ea51b042c24a07bd9)

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>

Python 3.12 backport: update also `test_nntplib`.

Co-authored-by: Nikita Sobolev <mail@sobolevn.me>
Co-authored-by: T. Wouters <thomas@python.org>