Feature request: TLS 1.3 support

[gilesorr]

I wanted to move some back end servers to run on TLS 1.3 only (our visitors accesses them through a re-encrypting load balancer that supports other TLS versions). I found that our Pingdom tests failed as soon as the TLS version changed: they don't support TLS 1.3 yet. That was enough of a push to try Cabot (which in many ways looks like a better solution for us ...) but it turns out that you don't support TLS 1.3 either. The test failed with: "Request error occurred: [SSL: TLSV1_ALERT_PROTOCOL_VERSION] tlsv1 alert protocol version (_ssl.c:661)" as soon as I changed the TLS version. It fixed itself and reported the test good when I supported TLS 1.2 again.

Please support TLS 1.3: it's been a valid spec for a year and a half, and provides better security than the previous standards.