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_tarfile.py b/Lib/test/test_tarfile.py
index 38a5027..034765c 100644
--- a/Lib/test/test_tarfile.py
+++ b/Lib/test/test_tarfile.py
@@ -703,7 +703,7 @@
         # Return True if the platform knows the st_blocks stat attribute and
         # uses st_blocks units of 512 bytes, and if the filesystem is able to
         # store holes in files.
-        if sys.platform == "linux2":
+        if sys.platform.startswith("linux"):
             # Linux evidentially has 512 byte st_blocks units.
             name = os.path.join(TEMPDIR, "sparse-test")
             with open(name, "wb") as fobj: