bpo-37583: Add err 113 to support.get_socket_conn_refused_errs() (GH-15259)

Add error number 113 EHOSTUNREACH to get_socket_conn_refused_errs()
of test.support.

Lib/test/support/__init__.py

@@ -1423,6 +1423,9 @@ def get_socket_conn_refused_errs():
         # bpo-31910: socket.create_connection() fails randomly
         # with EADDRNOTAVAIL on Travis CI
         errors.append(errno.EADDRNOTAVAIL)
+    if hasattr(errno, 'EHOSTUNREACH'):
+        # bpo-37583: The destination host cannot be reached
+        errors.append(errno.EHOSTUNREACH)
     return errors