Mark this test as flaky (#678)

diff --git a/tests/test_crypto.py b/tests/test_crypto.py
index 2f8a70d..2e102c9 100644
--- a/tests/test_crypto.py
+++ b/tests/test_crypto.py
@@ -1664,6 +1664,7 @@
         with pytest.raises(TypeError):
             cert.gmtime_adj_notAfter(None)
 
+    @flaky.flaky
     def test_gmtime_adj_notAfter(self):
         """
         `X509.gmtime_adj_notAfter` changes the not-after timestamp