Code extremely simplified:
---
import unittest
import sys
import _imp

class ModuleSpec:
    pass

class Tests(unittest.TestCase):
    def test_import_fresh(self):
        spec = ModuleSpec()
        spec.sys_weakref = sys.modules["_weakref"]
        spec.name = "_weakref"
        module = _imp.create_builtin(spec)
        module.__spec__ = spec
        sys.modules["_weakref"] = module
---

I still get a leak:
---
test_leak leaked [34, 34, 34] references, sum=102
test_leak leaked [11, 11, 11] memory blocks, sum=33
---

I understand that sys.modules["_weakref"] is overriden by the test with a new module, and the new module holds a reference somehow to the previous _weakref module. The old and the new modules remain alive.