remove py35 compatibility code

as f-strings are heavily used now, compatibility to py35
can be removed

Change-Id: I1ae4912ad4cbde8419b74845217943bd061053f3
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/32754
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

frappy/protocol/interface/tcp.py

@@ -231,13 +231,6 @@ class TCPServer(DualStackTCPServer):
             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()
 
 def format_address(addr):
     if len(addr) == 2: