Move PyOpenSSL-backed SSL into contrib.
Tests for urllib3.contrib.pyopenssl.

This makes the SNI-test raise SkipTest if SNI is not supported, so
it can be reused by test.contrib.test_pyopenssl even when the stdlib
does not support SNI.

It also tests the urllib3.contrib.pyopenssl module when its
dependencies are installed, by re-exporting HTTPS test-cases and
using module-level setup/teardown code to monkey patch urllib3.