Add an actual unit test for the lightweight type inference.

Author: Guido van Rossum

test-data/unit/check-modules.test

@@ -815,9 +815,12 @@ x = 0
 -- Test stability under import cycles
 -- ----------------------------------
 
--- The two tests are identical except one main has 'import x' and the other 'import y'.
+-- The first two tests are identical except one main has 'import x' and the other 'import y'.
 -- Previously (before build.order_ascc() was added) one of these would fail because the
 -- imports were processed in the (reverse) order in which the files were encountered.
+--
+-- The third one isn't fixed by order_ascc(), but is fixed by the lightweight type
+-- inference added to semanal.py (analyze_simple_literal_type()).
 
 [case testImportCycleStability1]
 import x
@@ -846,3 +849,17 @@ import x
 class Sub(x.Base):
     attr = x.Base.attr
 [out]
+
+[case testImportCycleStability3]
+import y
+[file x.py]
+class Base:
+    pass
+def foo() -> int:
+    import y
+    return y.Sub.attr
+[file y.py]
+import x
+class Sub(x.Base):
+    attr = 0
+[out]