Failure in test_defaultdict during CI

Saw the following failure during CI for netcoreapp2.1.CPython.test_defaultdict:

  Error Message:
   Traceback (most recent call last):
  File "D:\a\1\s\Src\StdLib\Lib\test\test_defaultdict.py", line 190, in <module>
  File "D:\a\1\s\Src\StdLib\Lib\test\test_defaultdict.py", line 187, in test_main
  File "D:\a\1\s\Src\StdLib\Lib\test\support\__init__.py", line 1787, in _run_suite
  File "D:\a\1\s\Src\StdLib\Lib\test\support\__init__.py", line 1812, in run_unittest
test.support.TestFailed: Traceback (most recent call last):
  File "D:\a\1\s\Src\StdLib\Lib\test\test_defaultdict.py", line 183, in test_pickleing
    o = pickle.loads(s)
  File "D:\a\1\s\Src\StdLib\Lib\pickle.py", line 1558, in _loads
    return _Unpickler(file, fix_imports=fix_imports,
  File "D:\a\1\s\Src\StdLib\Lib\pickle.py", line 1041, in load
    dispatch[key[0]](self)
  File "D:\a\1\s\Src\StdLib\Lib\pickle.py", line 1336, in load_global
    klass = self.find_class(module, name)
  File "D:\a\1\s\Src\StdLib\Lib\pickle.py", line 1386, in find_class
    __import__(module, level=0)
ImportError: No module named '__builtin__'

Have not been able to reproduce.

Start with Src/StdLib/Lib/test/test_defaultdict.py, especially test_pickleing, and the failing path in Src/StdLib/Lib/pickle.py. Run the netcoreapp2.1.CPython.test_defaultdict test or its local equivalent to investigate the reported builtin import failure. Done means the failure is reproduced or explained and the relevant test passes in CI.