db_default with decimal.Decimal() doesn't return Decimal instances on Oracle.

[Mariusz Felisiak]

While working on #34936 I noticed that DecimalField with db_default set to a Decimal() instance doesn't return Decimal() on Oracle:

/runtests.py field_defaults
Testing against Django installed in '/django/django' with up to 8 processes
Found 16 test(s).
Creating test database for alias 'default'...
Creating test user...
System check identified no issues (0 silenced).
.....s..sF......
======================================================================
FAIL: test_field_db_defaults_returning (field_defaults.tests.DefaultTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/django/django/test/testcases.py", line 1427, in skip_wrapper
    return test_func(*args, **kwargs)
  File "/django/tests/field_defaults/tests.py", line 48, in test_field_db_defaults_returning
    self.assertEqual(a.cost, Decimal("3.33"))
AssertionError: 3.33 != Decimal('3.33')

----------------------------------------------------------------------
Ran 16 tests in 0.089s

FAILED (failures=1, skipped=2)
Destroying test database for alias 'default'...
Destroying test user...
Destroying test database tables...

Regression in 7414704e88d73dafbcfbb85f9bc54cb6111439d3.