[2.7] bpo-35925: Skip SSL tests that fail due to weak external certs or old TLS (GH-13124) (GH-13253)

Modern Linux distros such as Debian Buster have default OpenSSL system
configurations that reject connections to servers with weak certificates
by default. This causes our test suite run with external networking
resources enabled to skip these tests when they encounter such a
failure.

Fixing the network servers is a separate issue.
(cherry picked from commit 2cc0223f)

Changes to test_ssl.py required as 2.7 has legacy protocol tests.

The test_httplib.py change is omitted from this backport as
self-signed.pythontest.net's certificate was updated and the
test_nntplib.py change is not applicable on 2.7.

Authored-by: Gregory P. Smith greg@krypto.org