bpo-38271: encrypt private key test files with AES256 (GH-16385)

bpo-38271: encrypt private key test files with AES256 (GH-16385)

The private keys for test_ssl were encrypted with 3DES in traditional
PKCSGH-5 format. 3DES and the digest algorithm of PKCSGH-5 are blocked by
some strict crypto policies. Use PKCSGH-8 format with AES256 encryption
instead.

Signed-off-by: Christian Heimes <christian@python.org>
https://bugs.python.org/issue38271

Automerge-Triggered-By: @tiran
(cherry picked from commit bfd0c963d88f3df69489ee250655e2b8f3d235bd)

Co-authored-by: Christian Heimes <christian@python.org>