Development infrastructure like Jenkins has no business being on the internet, so my preferred "second factor" is a VPN, secured with machine specific certificates that offer only VPN connectivity, but not SSH or anything else. This means even if a developer's Git or SSH key is floating around your infrastructure no one without VPN access can get at it, and if a developer loses a laptop there's a good chance you'll hear about that in time to revoke that machine's VPN certificate before anyone has a chance to use it.