Address reference leak issue (fixes #1029)

Creating an instance of of a pybind11-bound type caused a reference leak in the
associated Python type object, which could prevent these from being collected
upon interpreter shutdown. This commit fixes that issue for all types that are
defined in a scope (e.g. a module). Unscoped anonymous types (e.g. custom
iterator types) always retain a positive reference count to prevent their
collection.

tests/test_class.py

@@ -202,3 +202,24 @@ def test_brace_initialization():
     a = m.BraceInitialization(123, "test")
     assert a.field1 == 123
     assert a.field2 == "test"
+
+
+@pytest.unsupported_on_pypy
+def test_class_refcount():
+    """Instances must correctly increase/decrease the reference count of their types (#1029)"""
+    from sys import getrefcount
+
+    class PyDog(m.Dog):
+        pass
+
+    for cls in m.Dog, PyDog:
+        refcount_1 = getrefcount(cls)
+        molly = [cls("Molly") for _ in range(10)]
+        refcount_2 = getrefcount(cls)
+
+        del molly
+        pytest.gc_collect()
+        refcount_3 = getrefcount(cls)
+
+        assert refcount_1 == refcount_3
+        assert refcount_2 > refcount_1