* Add skip when required TLS version unsupported
Allow tests to skip, rather than fail from SSL errors with unsupported
TLS version, e.g. if Openssl is configured with MinProtocol higher than
the version being tested.
* Regenerate test certificate and key
Regenerate these with more secure settings for the sake of future
proofing, regenerate the key using RSA 4096 and sign the certificate
with SHA512.
This fixes test failures in tests/test_ssl.py when the user's OpenSSL
security level is set to a value greater than 1 and resolves issue #948
* Suppress SSL warnings in no verify tests
Some of the tests now use the `httpbin_both` fixture from pytest-httpbin.
Also, made httpbin's CA trusted by default and added `httpbin_secure_untrusted`
fixture to allow overriding that for particular tests.
