Reify object from base type

The currect assumption that `object` can be used to create a new object
is not always valid. One example is the issue I'm currently facing:
```
>>> reify(ast.Num(n=var('x')), {var('x'): 123})
...
     55 def _reify_object_dict(o, s):
---> 56     obj = object.__new__(type(o))
     57
     58     d = yield _reify(o.__dict__, s)

TypeError: object.__new__(Num) is not safe, use Num.__new__()
```

tests/test_more.py

@@ -1,3 +1,4 @@
+from ast import Num
 from collections.abc import Mapping
 
 from unification import var
@@ -29,6 +30,13 @@ def test_unify_object():
     assert stream_eval(_unify_object(Foo(1, 2), Foo(1, x), {})) == {x: 2}
 
 
+def test_unify_nonstandard_object():
+    x = var()
+    assert stream_eval(_unify_object(Num(n=1), Num(n=1), {})) == {}
+    assert stream_eval(_unify_object(Num(n=1), Num(n=2), {})) is False
+    assert stream_eval(_unify_object(Num(n=1), Num(n=x), {})) == {x: 1}
+
+
 def test_reify_object():
     x = var()
     obj = stream_eval(_reify_object(Foo(1, x), {x: 4}))

unification/more.py

@@ -53,7 +53,7 @@ def _reify_object(o, s):
 
 
 def _reify_object_dict(o, s):
-    obj = object.__new__(type(o))
+    obj = type(o).__new__(type(o))
 
     d = yield _reify(o.__dict__, s)