Add a test of evaluating a type with a link in its attrs (#17)

I was poking around at #13, thinking about to what extent the various
data structures there can be unified, and noticed that no tests failed
if I got rid of `seen`!

That seemed wrong, so I wrote some tests to catch those
cases. (Without seen, we blow the stack.)

Note that in these cases (of classes), not only do we want to not blow
the stack, but we would like the recursively referenced type to be
identical to the top-level one.  (Whereas I think for *type aliases*,
I think that is not what we want, since it probably wouldn't pretty
print correctly?)

Author: msullivan

tests/test_type_dir.py

@@ -154,6 +154,37 @@ class Final(Mine, Ordinary, Wrapper[float], AnotherBase[float], Last[int]):
 ]
 
 
+# Subtyping this forces real type evaluation
+class Eval[T]:
+    pass
+
+
+class Loop(Eval[int]):
+    loop: Loop
+
+
+class Foo(Eval[int]):
+    bar: Bar
+
+
+class Bar(Eval[float]):
+    foo: Foo
+
+
+def test_type_dir_link_1():
+    d = eval_typing(Loop)
+    loop = d.__annotations__["loop"]
+    assert loop is d
+    assert loop is not Foo
+
+
+def test_type_dir_link_2():
+    d = eval_typing(Foo)
+    loop = d.__annotations__["bar"].__annotations__["foo"]
+    assert loop is d
+    assert loop is not Foo
+
+
 def test_type_dir_1():
     d = eval_typing(Final)