asyncio: Move loop attribute to _FlowControlMixin

Move the _loop attribute from the constructor of _SelectorTransport,
_ProactorBasePipeTransport and _UnixWritePipeTransport classes to the
constructor of the _FlowControlMixin class.

Add also an assertion to explicit that the parent class must ensure that the
loop is defined (not None)

Lib/test/test_asyncio/test_transports.py

@@ -69,7 +69,8 @@ class TransportTests(unittest.TestCase):
             def get_write_buffer_size(self):
                 return 512
 
-        transport = MyTransport()
+        loop = mock.Mock()
+        transport = MyTransport(loop=loop)
         transport._protocol = mock.Mock()
 
         self.assertFalse(transport._protocol_paused)