py/objtype: Add __dict__ attribute for class objects.

The behavior mirrors the instance object dict attribute where a copy of the
local attributes are provided (unless the dict is read-only, then that dict
itself is returned, as an optimisation).  MicroPython does not support
modifying this dict because the changes will not be reflected in the class.

The feature is only enabled if MICROPY_CPYTHON_COMPAT is set, the same as
the instance version.

tests/basics/class_dict.py

@@ -0,0 +1,19 @@
+# test __dict__ attribute of a class
+
+if not hasattr(int, "__dict__"):
+    print("SKIP")
+    raise SystemExit
+
+
+# dict of a built-in type
+print("from_bytes" in int.__dict__)
+
+
+# dict of a user class
+class Foo:
+    a = 1
+    b = "bar"
+
+
+d = Foo.__dict__
+print(d["a"], d["b"])