fix python 3.5 compatibility

- move workaround for PEP487 ty secop.lib.py35compat
- add missing context manager to TCPServer
- remove redundant code in secop/property.py

Change-Id: I0822010f196ad6cec5ec44e990013b79c5d4048b
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/27090
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>

secop/protocol/interface/tcp.py

@@ -202,3 +202,11 @@ class TCPServer(socketserver.ThreadingTCPServer):
         if ntry:
             self.log.warning('tried again %d times after "Address already in use"' % ntry)
         self.log.info("TCPServer initiated")
+
+    # py35 compatibility
+    if not hasattr(socketserver.ThreadingTCPServer, '__exit__'):
+        def __enter__(self):
+            return self
+
+        def __exit__(self, *args):
+            self.server_close()