Add socket enum classes from py3.4+ (#1121)

* Add socket enum classes from py3.4+

Adds four IntEnum classes in the socket module that mirror the
AF_, AI_, MSG_, and SOCK_ sets of constants.

* Update socket AddressInfo/MsgFlag to use IntFlag type

* IntFlag, AddressInfo, and MsgFlag are py 3.6+

stdlib/3/socket.pyi

@@ -5,6 +5,7 @@
 # see: http://hg.python.org/cpython/file/3d0686d90f55/Lib/socket.py
 # see: http://nullege.com/codes/search/socket
 
+import sys
 from typing import Any, Tuple, List, Optional, Union, overload
 
 # ----- variables and constants -----
@@ -250,6 +251,73 @@ TIPC_WITHDRAWN = 0
 TIPC_ZONE_SCOPE = 0
 
 
+# enum versions of above flags py 3.4+
+if sys.version_info >= (3, 4):
+    from enum import IntEnum
+
+    class AddressFamily(IntEnum):
+        AF_UNIX = ...
+        AF_INET = ...
+        AF_INET6 = ...
+        AF_APPLETALK = ...
+        AF_ASH = ...
+        AF_ATMPVC = ...
+        AF_ATMSVC = ...
+        AF_AX25 = ...
+        AF_BLUETOOTH = ...
+        AF_BRIDGE = ...
+        AF_DECnet = ...
+        AF_ECONET = ...
+        AF_IPX = ...
+        AF_IRDA = ...
+        AF_KEY = ...
+        AF_LLC = ...
+        AF_NETBEUI = ...
+        AF_NETLINK = ...
+        AF_NETROM = ...
+        AF_PACKET = ...
+        AF_PPPOX = ...
+        AF_ROSE = ...
+        AF_ROUTE = ...
+        AF_SECURITY = ...
+        AF_SNA = ...
+        AF_TIPC = ...
+        AF_UNSPEC = ...
+        AF_WANPIPE = ...
+        AF_X25 = ...
+
+    class SocketKind(IntEnum):
+        SOCK_STREAM = ...
+        SOCK_DGRAM = ...
+        SOCK_RAW = ...
+        SOCK_RDM = ...
+        SOCK_SEQPACKET = ...
+        SOCK_CLOEXEC = ...
+        SOCK_NONBLOCK = ...
+
+if sys.version_info >= (3, 6):
+    from enum import IntFlag
+
+    class AddressInfo(IntFlag):
+        AI_ADDRCONFIG = ...
+        AI_ALL = ...
+        AI_CANONNAME = ...
+        AI_NUMERICHOST = ...
+        AI_NUMERICSERV = ...
+        AI_PASSIVE = ...
+        AI_V4MAPPED = ...
+
+    class MsgFlag(IntFlag):
+        MSG_CTRUNC = ...
+        MSG_DONTROUTE = ...
+        MSG_DONTWAIT = ...
+        MSG_EOR = ...
+        MSG_OOB = ...
+        MSG_PEEK = ...
+        MSG_TRUNC = ...
+        MSG_WAITALL = ...
+
+
 # ----- exceptions -----
 class error(IOError):
     ...