py/objmodule: Add support for __dict__.

This matches class `__dict__`, and is similarly gated on
MICROPY_CPYTHON_COMPAT. Unlike class though, because modules's globals are
actually dict instances, the result is a mutable dictionary.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

tests/basics/module_dict.py

@@ -0,0 +1,13 @@
+# test __dict__ attribute of a built-in module
+# see import/module_dict.py for the equivalent test on user modules
+
+import sys
+
+if not hasattr(sys, "__dict__"):
+    print("SKIP")
+    raise SystemExit
+
+
+# dict of a built-in module (read-only)
+print(type(sys.__dict__))
+print(sys.__dict__["__name__"])