test: Make tests having yield from only with Python 3

If Python is older than 3.3, "yield from" will be SyntaxError like PR
riptideio#400 with "SyntaxError: invalid syntax" [1]. It is defined in
PEP 380 -- Syntax for Delegating to a Subgenerator [2]:

    __import__(modname)
E     File "/home/travis/build/riptideio/pymodbus/test/test_server_asyncio.py", line 74
E       server = yield from StartTcpServer(context=self.context,loop=self.loop,identity=identity)
E                         ^
E   SyntaxError: invalid syntax

This patch skips the tests if they are not in Python 3.

[1] https://travis-ci.org/riptideio/pymodbus/jobs/584493268
[2] https://www.python.org/dev/peps/pep-0380/

test/test_server_asyncio.py

@@ -93,6 +93,7 @@ def testTcpServerServeForever(self):
             yield from server.serve_forever()
             serve.assert_awaited()
 
+    @pytest.mark.skipif(not IS_PYTHON3, reason="requires python3.4 or above")
     @asyncio.coroutine
     def testTcpServerServeForeverTwice(self):
         ''' Call on serve_forever() twice should result in a runtime error '''
@@ -106,6 +107,7 @@ def testTcpServerServeForeverTwice(self):
             yield from server.serve_forever()
         server.server_close()
 
+    @pytest.mark.skipif(not IS_PYTHON3, reason="requires python3.4 or above")
     @asyncio.coroutine
     def testTcpServerReceiveData(self):
         ''' Test data sent on socket is received by internals - doesn't not process data '''
@@ -140,6 +142,7 @@ def eof_received(self):
             self.assertTrue( process.call_args[1]["data"] == data )
             server.server_close()
 
+    @pytest.mark.skipif(not IS_PYTHON3, reason="requires python3.4 or above")
     @asyncio.coroutine
     def testTcpServerRoundtrip(self):
         ''' Test sending and receiving data on tcp socket '''
@@ -180,6 +183,7 @@ def eof_received(self):
         yield from asyncio.sleep(0)
         server.server_close()
 
+    @pytest.mark.skipif(not IS_PYTHON3, reason="requires python3.4 or above")
     @asyncio.coroutine
     def testTcpServerConnectionLost(self):
         ''' Test tcp stream interruption '''
@@ -213,6 +217,7 @@ def connection_made(self, transport):
         self.assertTrue( len(server.active_connections) == 0 )
         server.server_close()
 
+    @pytest.mark.skipif(not IS_PYTHON3, reason="requires python3.4 or above")
     @asyncio.coroutine
     def testTcpServerCloseActiveConnection(self):
         ''' Test server_close() while there are active TCP connections '''
@@ -245,6 +250,7 @@ def connection_made(self, transport):
         yield from asyncio.sleep(0.0)
         self.assertTrue( len(server.active_connections) == 0 )
 
+    @pytest.mark.skipif(not IS_PYTHON3, reason="requires python3.4 or above")
     @asyncio.coroutine
     def testTcpServerException(self):
         ''' Sending garbage data on a TCP socket should drop the connection '''
@@ -283,6 +289,7 @@ def eof_received(self):
             # neither of these should timeout if the test is successful
             server.server_close()
 
+    @pytest.mark.skipif(not IS_PYTHON3, reason="requires python3.4 or above")
     @asyncio.coroutine
     def testTcpServerNoSlave(self):
         ''' Test unknown slave unit exception '''
@@ -319,6 +326,7 @@ def eof_received(self):
         self.assertFalse(eof.done())
         server.server_close()
 
+    @pytest.mark.skipif(not IS_PYTHON3, reason="requires python3.4 or above")
     @asyncio.coroutine
     def testTcpServerModbusError(self):
         ''' Test sending garbage data on a TCP socket should drop the connection '''
@@ -358,6 +366,7 @@ def eof_received(self):
             transport.close()
             server.server_close()
 
+    @pytest.mark.skipif(not IS_PYTHON3, reason="requires python3.4 or above")
     @asyncio.coroutine
     def testTcpServerInternalException(self):
         ''' Test sending garbage data on a TCP socket should drop the connection '''
@@ -404,6 +413,7 @@ def eof_received(self):
     # Test ModbusUdpProtocol
     #-----------------------------------------------------------------------#
 
+    @pytest.mark.skipif(not IS_PYTHON3, reason="requires python3.4 or above")
     @asyncio.coroutine
     def testStartUdpServer(self):
         ''' Test that the modbus udp asyncio server starts correctly '''
@@ -430,6 +440,7 @@ def testUdpServerServeForeverStart(self):
             yield from server.serve_forever()
             serve.assert_awaited()
 
+    @pytest.mark.skipif(not IS_PYTHON3, reason="requires python3.4 or above")
     @asyncio.coroutine
     def testUdpServerServeForeverClose(self):
         ''' Test StartUdpServer serve_forever() method '''
@@ -446,6 +457,7 @@ def testUdpServerServeForeverClose(self):
         server.server_close()
         self.assertTrue(server.protocol.is_closing())
 
+    @pytest.mark.skipif(not IS_PYTHON3, reason="requires python3.4 or above")
     @asyncio.coroutine
     def testUdpServerServeForeverTwice(self):
         ''' Call on serve_forever() twice should result in a runtime error '''
@@ -461,6 +473,7 @@ def testUdpServerServeForeverTwice(self):
             yield from server.serve_forever()
         server.server_close()
 
+    @pytest.mark.skipif(not IS_PYTHON3, reason="requires python3.4 or above")
     @asyncio.coroutine
     def testUdpServerReceiveData(self):
         ''' Test that the sending data on datagram socket gets data pushed to framer '''
@@ -482,6 +495,7 @@ def testUdpServerReceiveData(self):
 
             server.server_close()
 
+    @pytest.mark.skipif(not IS_PYTHON3, reason="requires python3.4 or above")
     @asyncio.coroutine
     def testUdpServerSendData(self):
         ''' Test that the modbus udp asyncio server correctly sends data outbound '''
@@ -524,6 +538,7 @@ def datagram_received(self, data, addr):
         self.assertTrue(server.protocol.is_closing())
         yield from asyncio.sleep(0.1)
 
+    @pytest.mark.skipif(not IS_PYTHON3, reason="requires python3.4 or above")
     @asyncio.coroutine
     def testUdpServerRoundtrip(self):
         ''' Test sending and receiving data on udp socket'''
@@ -562,6 +577,7 @@ def datagram_received(self, data, addr):
         yield from asyncio.sleep(0)
         server.server_close()
 
+    @pytest.mark.skipif(not IS_PYTHON3, reason="requires python3.4 or above")
     @asyncio.coroutine
     def testUdpServerException(self):
         ''' Test sending garbage data on a TCP socket should drop the connection '''