skip NPN tests if NPN is not available (#822)

* skip NPN tests if NPN is not available

* use the right name
diff --git a/tests/test_ssl.py b/tests/test_ssl.py
index 986463a..ed911de 100644
--- a/tests/test_ssl.py
+++ b/tests/test_ssl.py
@@ -1737,6 +1737,9 @@
         assert args == [(server, b"foo1.example.com")]
 
 
+@pytest.mark.skipif(
+    not _lib.Cryptography_HAS_NEXTPROTONEG, reason="NPN is not available"
+)
 class TestNextProtoNegotiation(object):
     """
     Test for Next Protocol Negotiation in PyOpenSSL.