caucase.http: Silence ssl.OP_NO_{SSL,TLS}* deprecation warning.

caucase/http.py

@@ -279,6 +279,8 @@ def startServerThread(server):
   server_thread.daemon = True
   server_thread.start()
 
+TLSVersion = getattr(ssl, 'TLSVersion', None)
+
 def getSSLContext(
   key_len,
   threshold,
@@ -299,8 +301,12 @@ def getSSLContext(
   # SSL is used for client authentication, and is only required for very few
   # users on any given caucased. So make ssl_context even stricter than python
   # does.
-  # No TLSv1 and TLSv1.1, leaving (currently) only TLSv1.2
+  # No TLSv1 and TLSv1.1
+  if TLSVersion is None: # pragma: no cover
+    # BBB: py<3.7
     ssl_context.options |= ssl.OP_NO_TLSv1 | ssl.OP_NO_TLSv1_1
+  else: # pragma: no cover
+    ssl_context.minimum_version = TLSVersion.TLSv1_2
 
   # If a client wishes to use https for unauthenticated operations, that's
   # fine too.