This flag is totally wrong here, although for whatever reason it doesn't actually seem to cause any problems observable in the test suite
diff --git a/src/crypto/pkey.c b/src/crypto/pkey.c
index f78acf7..6632abe 100644
--- a/src/crypto/pkey.c
+++ b/src/crypto/pkey.c
@@ -231,7 +231,7 @@
     NULL, /* getattro */
     NULL, /* setattro */
     NULL, /* as_buffer */
-    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
+    Py_TPFLAGS_DEFAULT,
     crypto_PKey_doc, /* doc */
     NULL, /* traverse */
     NULL, /* clear */