bpo-38148: Add slots to asyncio transports (GH-16077)

* bpo-38148: Add slots to asyncio transports

* Update Misc/NEWS.d/next/Library/2019-09-13-08-55-43.bpo-38148.Lnww6D.rst

Co-Authored-By: Kyle Stanley <aeros167@gmail.com>

Lib/asyncio/transports.py

@@ -9,6 +9,8 @@ __all__ = (
 class BaseTransport:
     """Base class for transports."""
 
+    __slots__ = ('_extra',)
+
     def __init__(self, extra=None):
         if extra is None:
             extra = {}
@@ -44,6 +46,8 @@ class BaseTransport:
 class ReadTransport(BaseTransport):
     """Interface for read-only transports."""
 
+    __slots__ = ()
+
     def is_reading(self):
         """Return True if the transport is receiving."""
         raise NotImplementedError
@@ -68,6 +72,8 @@ class ReadTransport(BaseTransport):
 class WriteTransport(BaseTransport):
     """Interface for write-only transports."""
 
+    __slots__ = ()
+
     def set_write_buffer_limits(self, high=None, low=None):
         """Set the high- and low-water limits for write flow control.
 
@@ -154,10 +160,14 @@ class Transport(ReadTransport, WriteTransport):
     except writelines(), which calls write() in a loop.
     """
 
+    __slots__ = ()
+
 
 class DatagramTransport(BaseTransport):
     """Interface for datagram (UDP) transports."""
 
+    __slots__ = ()
+
     def sendto(self, data, addr=None):
         """Send data to the transport.
 
@@ -180,6 +190,8 @@ class DatagramTransport(BaseTransport):
 
 class SubprocessTransport(BaseTransport):
 
+    __slots__ = ()
+
     def get_pid(self):
         """Get subprocess id."""
         raise NotImplementedError
@@ -247,6 +259,8 @@ class _FlowControlMixin(Transport):
     resume_writing() may be called.
     """
 
+    __slots__ = ('_loop', '_protocol_paused', '_high_water', '_low_water')
+
     def __init__(self, extra=None, loop=None):
         super().__init__(extra)
         assert loop is not None