Assume SO_PROTOCOL and SO_DOMAIN exist on Linux

Since we no longer support Python 3.5.

Author: pquentin

trio/_socket.py

@@ -295,13 +295,7 @@ def _sniff_sockopts_for_fileno(family, type, proto, fileno):
     # and then we'll throw it away and construct a new one with the correct metadata.
     if not _sys.platform == "linux":
         return family, type, proto
-    try:
-        from socket import SO_DOMAIN, SO_PROTOCOL
-    except ImportError:
-        # Only available on 3.6 and above:
-        SO_PROTOCOL = 38
-        SO_DOMAIN = 39
-    from socket import SOL_SOCKET, SO_TYPE
+    from socket import SO_DOMAIN, SO_PROTOCOL, SOL_SOCKET, SO_TYPE
     sockobj = _stdlib_socket.socket(family, type, proto, fileno=fileno)
     try:
         family = sockobj.getsockopt(SOL_SOCKET, SO_DOMAIN)