Issue #12326: don't test the major version of sys.platform

Use startswith, instead of ==, when testing sys.platform to support
new platforms like Linux 3 or OpenBSD 5.
diff --git a/Lib/test/test_socket.py b/Lib/test/test_socket.py
index a00a99f..320f373 100644
--- a/Lib/test/test_socket.py
+++ b/Lib/test/test_socket.py
@@ -2074,7 +2074,7 @@
     ])
     if hasattr(socket, "socketpair"):
         tests.append(BasicSocketPairTest)
-    if sys.platform == 'linux2':
+    if sys.platform.startswith('linux'):
         tests.append(TestLinuxAbstractNamespace)
     if isTipcAvailable():
         tests.append(TIPCTest)