Add client support for python 3.5+

Starting with python 3.7 ssl.SSLSocket was removed. Simply calling wrap_socket is the proposed fix here.
Starting with python 3.5 ssl.PROTOCOL_TLSv1 was removed, so we switch to PROTOCOL_TLS, which should result in the highest possible TLS connection. Modern OSes do no longer support SSLv2/3 so it is okayish to ignore those.

This should fix Bcfg2#415

src/lib/Bcfg2/Client/Proxy.py

@@ -198,6 +198,9 @@ def connect(self):
             ssl_protocol_ver = ssl.PROTOCOL_SSLv23
         elif self.protocol == 'xmlrpc/tlsv1':
             ssl_protocol_ver = ssl.PROTOCOL_TLSv1
+        elif self.protocol == 'xmlrpc/tls':
+            # needed for python 3.5+ support
+            ssl_protocol_ver = ssl.PROTOCOL_TLS
         else:
             self.logger.error("Unknown protocol %s" % (self.protocol))
             raise Exception("unknown protocol %s" % self.protocol)
@@ -219,7 +222,7 @@ def connect(self):
             self.key = None
 
         rawsock.settimeout(self.timeout)
-        self.sock = ssl.SSLSocket(rawsock, cert_reqs=other_side_required,
+        self.sock = ssl.wrap_socket(rawsock, cert_reqs=other_side_required,
                                   ca_certs=self.ca, suppress_ragged_eofs=True,
                                   keyfile=self.key, certfile=self.cert,
                                   ssl_version=ssl_protocol_ver)