Avoid mucking with SSL module internals from Dulwich.

Before Python 2.7.9, the ssl module lacks SNI support and may lag
behind in security updates. The recommended approach by urllib3 is to
use pyOpenSSL as a replacement for those versions. It will
issue InsecurePlatformWarning and SNIMissingWarning when
run on old ssl module versions, allowing the user to take this action.

This avoids a hard dependency on pyopenssl and friends from Dulwich.