my.core.serialize/tox: move serialize tests

can't be in tests-core because it depends
on orjson being installed

tests/core.py

@@ -18,4 +18,3 @@ from my.core.util           import *
 from my.core.discovery_pure import *
 from my.core.types          import *
 from my.core.stats          import *
-from my.core.serialize      import *

tests/serialize.py

@@ -0,0 +1 @@
+from my.core.serialize      import *